1. Mail Services

2. Mail Services Three major mail services: Simple Mail Transfer Protocol
SMTP
Post Office Protocol
POP3
Internet Mail Access Protocol
IMAP or IMAP4

3. Which one to use? Outbound Mail Inbound Mail SMTP for outbound email
Port 25 or 2525
Inbound Mail
POP3 for inbound
Port 110
IMAP for inbound
Port 143

4. What is SMTP?
Simple Mail Transfer Protocol (SMTP) is the standard protocol for sending s across the Internet.
SMTP uses TCP port 25 or 2525
Sometimes you may have problems sending messages
ISP may have closed port 25
To determine the SMTP server for a given domain name
MX (Mail eXchange) DNS record is used

5. IMAP or POP3? Both receive email
Main differences between IMAP and POP3:
POP3 protocol assumes there is only one client connected to the mailbox
IMAP protocol allows simultaneous access by multiple clients
IMAP is suitable if the mailbox is used by multiple users

6. Material from: http://en.wikipedia.org/wiki/Smtp
SMTP
Material from:

7. SMTP Simple Mail Transfer Protocol
The de facto standard for transmissions across the Internet.
Defined in RFC 821 (STD 10)
amended by RFC 1123 (STD 3) chapter 5.
The protocol used today is also known as ESMTP
Defined in RFC 2821.

8. SMTP Relatively simple text-based protocol Client-server protocol
One or more recipients of a message are specified
In most cases verified to exist
The message text is transferred
Client-server protocol
The client transmits an message to the server
Either
an end-user's client, a.k.a. MUA (Mail User Agent), - or -
a relaying server's MTA (Mail Transfer Agents)
can act as an SMTP client.

9. SMTP
An client knows the outgoing mail SMTP server from its configuration
A relaying server typically determines which SMTP server to connect to by looking up the MX (Mail eXchange) DNS record for each recipient's domain name
The part of the address to the right of the at sign
Conformant MTAs (not all) fall back to a simple A record in the case of no MX
A Record: address record
Some current mail transfer agents will also use SRV records
SRV Records: SeRVice records
More general form of MX,
These are not widely adopted.
Relaying servers can also be configured to use a smart host

10. SMTP
The SMTP client initiates a TCP connection to the server's port 25
unless overridden by configuration
It is quite easy to test an SMTP server using the telnet program
see following example

11. SMTP
SMTP is a "push" protocol that does not allow one to "pull" messages from a remote server on demand
That is it sends only
To do a pull (i.e. receive) a mail client must use POP3 or IMAP
Another SMTP server can trigger a delivery in SMTP using ETRN

12. Outgoing mail SMTP server
An client requires the name or the IP address of an SMTP server as part of its configuration
Server will deliver messages on behalf of the user
Setting allows for various policies and network designs
End users connected to the Internet can use the services of an provider that is not necessarily the same as their connection provider
Network topology, or the location of a client within a network or outside of a network, is no longer a limiting factor for submission or delivery
Modern SMTP servers typically use a client's credentials (authentication) rather than a client's location (IP address), to determine whether it is eligible to relay .

13. Outgoing mail SMTP server
Another choice is whether to use TCP port 25 (SMTP) or port 587 (Submission), as established by RFC 2476, for relaying outbound mail to a mail server.
Many servers support both.
Some servers still support port 465 for legacy secure SMTP
It is preferable to use encryption on standard ports according to RFC 2487.
Some servers are setup to reject all relaying on port 25
Valid users authenticating on port 587 are allowed to relay mail to any valid address.
A server that relays all for all destinations for all clients connecting to port 25 is known as an open relay
Generally considered a bad practice worthy of blacklisting.

14. Sample communications
After establishing a connection between the sender (the client) and the receiver (the server), the following page shows a legal SMTP session.
In the following conversation, everything sent by the client is prefaced with C: and everything sent by the server is prefaced with S:.
On most computer systems, a connection can be established using the telnet command on the client machine, for example:
telnet 25
Opens a TCP connection from the sending machine to the MTA listening on port 25 on host

15. Sample communications
S: ESMTP Postfix
C: HELO mydomain.com
S: 250 Hello mydomain.com
C: MAIL
S: 250 Ok
C: RCPT
C: DATA
S: 354 End data with <CR><LF>.<CR><LF>
C: Subject: test message
C: From:
C: To: C:
C: Hello,
C: This is a test.
C: Goodbye.
C: .
S: 250 Ok: queued as 12345
C: QUIT
S: 221 Bye

16. Sample communications
Note: the data the client sends in the HELO and MAIL FROM commands can be retrieved in additional headers that the server adds to the message:
Received and Return-Path respectively.
Optional: (and not shown above) nearly all clients ask the server which SMTP extensions the server supports
Uses the EHLO greeting to invoke Extended SMTP (ESMTP).
These clients use HELO only if the server does not respond to EHLO.

17. Sample communications
Contemporary clients will use the ESMTP extension keyword SIZE to inquire of the server the maximum message size that will be accepted.
Older clients and servers
Try to transfer huge messages that will be rejected
After wasting the network resources
Including a lot of connect time to dialup ISPs
paid by the minute.

18. Sample communications
For the edit planning of giant files or sending with older clients, users can manually determine in advance the maximum size accepted by ESMTP servers.
The user telnets as above, but substitutes "EHLO mydomain.com" for the HELO command line:
S: 220-serverdomain.com ESMTP {postfix version and date}
S: 220 NO UCE. {etc., terms of service}
C: EHLO mydomain.com
S: 250-serverdomain.com Hello mydomain.com [ ]
S: 250-SIZE
S: 250-PIPELINING
S: 250 HELP
This serverdomain.com declares that it will accept a fixed maximum message size no larger than 14,680,064 octets (8-bit bytes).
Depending on the server's actual resource usage, it may be currently unable to accept a message this large.

19. Sample communications
In the simplest case, an ESMTP server will declare a maximum SIZE with only the EHLO user interaction.
If no number appears after the SIZE keyword, or if the current message limit must be exactly determined:
User can further interact by simulating the ESMTP header of a message with an estimated size.
See External Link RFC 1870

20. From: http://en.wikipedia.org/wiki/Post_Office_Protocol
POP
From:

21. POP Local e-mail clients use the Post Office Protocol version 3 (POP3)
An application-layer Internet standard protocol
Retrieves from a remote server over a TCP/IP connection
Many subscribers to individual Internet service provider accounts access their with client software that uses POP3.

22. POP Overview POP3 has made earlier versions of the protocol obsolete
POP (informally called POP1 and POP2)
Now, the term POP almost always means POP3 in the context of protocols

23. POP Overview
The design of POP3 and its procedures supports end-users with intermittent connections (such as dial-up connections)
Allows users to retrieve when connected
View and manipulate the retrieved messages without needing to stay connected
Although most clients have an option to leave mail on server, clients using POP3 generally:
Connect
Retrieve all messages
Store them on the user's PC as new messages
Delete them from the server
Disconnect.

24. POP Overview
In contrast, the newer, more capable Internet Message Access Protocol (IMAP) supports both connected and disconnected modes of operation.
clients using IMAP generally leave messages on the server until the user explicitly deletes them.
This and other facets of IMAP operation allow multiple clients to access the same mailbox

25. POP Overview
Most clients support either POP3 or IMAP to retrieve messages
Few Internet Service Providers (ISPs) support IMAP
The fundamental difference between POP3 and IMAP4:
POP3 offers access to a mail drop
Mail exists on the server until it is collected by the client
If the client leaves some or all messages on the server
The client's message store is considered authoritative
In contrast, IMAP4 offers access to the mail store
The client may store local copies of the messages
These are considered to be a temporary cache
The server's store is authoritative

26. POP Overview
Clients with a leave mail on server option generally use the POP3 UIDL (Unique IDentification Listing) command.
Most POP3 commands identify specific messages by their ordinal number on the mail server.
Creates a problem for a client intending to leave messages on the server
Message numbers may change from one connection to the server to another
For example if a mailbox contains five messages at last connect, and a different client then deletes message #3, the next connecting user will find the last two messages' numbers decremented by one
UIDL provides a mechanism to avoid these numbering issues.
Server assigns a string of characters as a permanent and unique ID for the message
When a POP3-compatible client connects to the server, it can use the UIDL command to get the current mapping from these message IDs to the ordinal message numbers
The client can then use this mapping to determine which messages it has yet to download
Saves time when downloading
Note: IMAP has a similar mechanism, using a 32-bit UID (Unique IDentifier) that is required to be strictly ascending.
The advantage of the numeric UID is with large mailboxes
A client can request just the UIDs greater than its previously stored "highest UID"
In POP, the client must fetch the entire UIDL map

27. POP Overview Whether using POP3 or IMAP to retrieve messages:
clients typically use the SMTP_Submit profile of the SMTP protocol to send messages
clients are commonly categorized as either POP or IMAP clients, but in both cases the clients also use SMTP
There are extensions to POP3 that allow some clients to transmit outbound mail via POP3
Known as "XTND XMIT" extensions.
The Qualcomm qpopper and CommuniGate Pro servers and Eudora clients are examples of systems that optionally utilize the XTND XMIT methods of authenticated client-to-server transmission.
MIME serves as the standard for attachments and non-ASCII text in
Although neither POP3 nor SMTP require MIME-formatted , essentially all Internet comes MIME-formatted
POP clients must also understand and use MIME
IMAP, by design, assumes MIME-formatted

28. POP Overview
POP3 originally supported only an unencrypted login mechanism
Although plain text transmission of passwords in POP3 still commonly occurs, POP3 currently supports several authentication methods to provide varying levels of protection against illegitimate access to a user's
One such method, APOP, uses the MD5 hash function in an attempt to avoid replay attacks and disclosure of the shared secret
Clients implementing APOP include Mozilla Thunderbird, Opera, Eudora, KMail and Novell Evolution
POP3 clients can also support SASL authentication methods via the AUTH extension. MIT Project Athena also produced a Kerberized version
POP3 works over a TCP/IP connection using TCP on network port 110
clients can encrypt POP3 traffic using TLS or SSL
A TLS or SSL connection is negotiated using the STLS command
Some clients and servers, like Google Gmail, instead use the deprecated alternate-port method, which uses TCP port 995

29. POP Example (APOP) S: <wait for connection on TCP port 110>
C: <open connection>
S: +OK POP3 server ready
C: APOP mrose c4c9334bac560ecc979e58001b3e22fb
S: +OK mrose's maildrop has 2 messages (320 octets)
C: STAT
S: +OK 2 320
C: LIST S: +OK 2 messages (320 octets)
S: 1 120
S: 2 200
S: .
C: RETR 1
S: +OK 120 octets
S: <the POP3 server sends message 1>
S: . C: DELE 1
S: +OK message 1 deleted
C: RETR 2
S: +OK 200 octets
S: <the POP3 server sends message 2>
S: . C: DELE 2
S: +OK message 2 deleted
C: QUIT S: +OK dewey POP3 server signing off (maildrop empty)
C: <close connection> S: <wait for next connection>

30. POP Example
POP3 servers without the optional APOP command expect you to log in with the USER and PASS commands:
C: USER mrose
S: +OK User accepted
C: PASS mrosepass
S: +OK Pass accepted

31. IMAP

32. IMAP Internet Message Access Protocol
Commonly known as IMAP or IMAP4
An application layer Internet protocol
Operates on port 143
Allows a local client to access on a remote server
Previously called Internet Mail Access Protocol, Interactive Mail Access Protocol (RFC 1064), and Interim Mail Access Protocol)
Current version is IMAP version 4 revision 1
IMAP4rev1: defined by RFC 3501
IMAP4 and POP3 are the two most prevalent Internet standard protocols for retrieval
Virtually all modern clients and servers support both

33. IMAP IMAP supports both connected and disconnected modes of operation.
clients using IMAP generally leave messages on the server until the user explicitly deletes them.
This and other facets of IMAP operation allow multiple clients to access the same mailbox.
Most clients support either POP3 or IMAP
Fewer Internet Service Providers (ISPs) support IMAP.
IMAP4 offers access to the mail store
the client may store local copies of the messages
considered to be a temporary cache
the server's store is authoritative.

34. IMAP E-mail messages are generally sent to an e-mail server
Stores received messages in the recipient's mailbox
User later retrieves these messages with either a web browser or an client that uses one of a number of retrieval protocols.
Some clients and servers preferentially use vendor specific, typically proprietary protocols, most support the Internet standard protocols
SMTP for sending
POP3 and IMAP4 for retrieving
Allows interoperability with other servers and clients
Proprietary protocols:
Microsoft Outlook client when communicating with an Exchange server
IBM Notes client when communicating with a Domino server
Both products also support SMTP, POP3, and IMAP4.
Support for the Internet standard protocols
Allows other clients to access these servers
Allows the clients to be used with other servers
E.g. Qualcomm's Eudora or Mozilla Thunderbird

35. IMAP
clients can generally be configured to use either POP3 or IMAP4 to retrieve and in both cases use SMTP for sending.
Most programs can also use Lightweight Directory Access Protocol (LDAP) for directory services
Many (if not most) subscribers to individual ISP accounts access their with client software that uses POP3
IMAP is often used in large networks
For example, a college campus mail system.
IMAP allows users to access new messages instantly on their computers
The mail is stored on the network
With POP3, users either
download the to their computer
access it via the web.
Both methods take longer than IMAP
User must either download any new mail or "refresh" the page to see the new messages.

36. Advantages over POP3 Connected and disconnected modes of operation
When using POP3, clients typically connect to the server briefly, only as long as it takes to download new messages.
When using IMAP4, clients often stay connected as long as the user interface is active and download message content on demand.
For users with many or large messages, this IMAP4 usage pattern can result in faster response times.
Multiple clients simultaneously connected to the same mailbox
The POP3 protocol requires the currently connected client to be the only client connected to the mailbox.
The IMAP protocol specifically allows simultaneous access by multiple clients
Provides mechanisms for clients to detect changes made to the mailbox by other, concurrently connected, clients

37. Advantages over POP3 Access to MIME message parts and partial fetch
Nearly all internet is transmitted in MIME format, allowing messages to have a tree structure where the leaf nodes are any of a variety of single part content types and the non-leaf nodes are any of a variety of multipart types.
The IMAP4 protocol allows clients to separately retrieve any of the individual MIME parts and also to retrieve portions of either individual parts or the entire message.
These mechanisms allow clients to retrieve the text portion of a message without retrieving attached files or to stream content as it is being fetched.

38. Advantages over POP3 Message state information
flags (defined in the IMAP4 protocol clients) can keep track of message state
for example whether or not the message has been read, replied to, or deleted
Flags are stored on the server
Different clients accessing the same mailbox at different times can detect state changes made by other clients
POP3 provides no mechanism for clients to store such state information on the server
If a single user accesses a mailbox with two different POP3 clients
State information, such as whether a message has been accessed, cannot be synchronized between the clients
The IMAP4 protocol supports both pre-defined system flags and client defined keywords
System flags indicate state information such as whether a message has been read.
Keywords, (not supported by all IMAP servers), allow messages to be given one or more tags whose meaning is up to the client
Adding user created tags to messages is an operation supported by some Webmail services, such as Gmail
Generally not using IMAP.

39. Advantages over POP3 Multiple mailboxes on the server
IMAP4 clients can create, rename, and/or delete mailboxes (usually presented to the user as folders) on the server, and move messages between mailboxes.
Multiple mailbox support also allows servers to provide access to shared and public folders.
Server-side searches
IMAP4 provides a mechanism for a client to ask the server to search for messages meeting a variety of criteria
Avoids requiring clients to download every message in the mailbox in order to perform searches
Built-in extension mechanism
IMAP4 defines an explicit mechanism by which it may be extended.
Many extensions to the base protocol have been proposed and are in common use.
IMAP2bis did not have an extension mechanism
POP3 now has one defined by RFC 2449.

40. Disadvantages of IMAP IMAP remedies many of the shortcomings of POP
Inherently introduces additional complexity
Much of this complexity (e.g., multiple clients accessing the same mailbox at the same time) is compensated for by server-side workarounds
Maildir
Database backends

41. Disadvantages of IMAP
Unless the mail store and searching algorithms on the server are carefully implemented, a client can potentially consume large amounts of server resources when searching massive mailboxes.

42. Disadvantages of IMAP
IMAP4 clients need to explicitly request new message content
Potentially causing additional delays on slow connections such as those commonly used by mobile devices.
A private proposal, push IMAP, would extend IMAP to implement push by sending the entire message instead of just a notification.
However, push IMAP has not been generally accepted
Current IETF work has addressed the problem in other ways

43. Disadvantages of IMAP
Unlike some proprietary protocols which combine sending and retrieval operations, sending a message and saving a copy in a server-side folder with a base-level IMAP client requires transmitting the message content twice, once to SMTP for delivery and a second time to IMAP to store in a sent mail folder.
Remedied by a set of extensions defined by the IETF LEMONADE Working Group for mobile devices
POP3 servers don't support server-side folders so clients have no choice but to store sent items on the client.
Many IMAP clients can be configured to store sent mail in a client-side folder.
Courier Mail Server offers a non-standard method of sending using IMAP by copying an outgoing message to a dedicated outbox folder.

44. SMTP-AUTH
SMTP Authentication

45. SMTP Authentication SMTP-AUTH
Extension of the Simple Mail Transfer Protocol (SMTP)
Includes an authentication step
Client effectively logs in to the mail server during the process of sending mail
Servers which support SMTP-AUTH can usually be configured to require clients to use this extension, ensuring the true identity of the sender is known.
SMTP-AUTH is defined in RFC 2554.

46. SMTP Authentication SMTP-AUTH provides an access control mechanism
Can be used to allow legitimate users to relay mail while denying relay service to unauthorized users, such as spammers
Does not guarantee the authenticity of either the SMTP envelope sender or the "From:" header.
Spoofing, in which one sender masquerades as someone else, is possible even with SMTP-AUTH.

47. SMTP Authentication
SMTP-AUTH extension also allows one mail server to indicate to another that the sender has been authenticated when relaying mail
This requires the recipient server to trust the sending server
This aspect of SMTP-AUTH is rarely used on the Internet.
The recipient of an message cannot tell whether the sender was authenticated
Use of SMTP-AUTH is only a partial solution to spam.

48. SMTP Authentication
While SMTP-AUTH is generally a security improvement over unauthenticated SMTP, it can also introduce a weakness
If authenticated users are allowed to submit messages from IP addresses where unauthenticated users are not — that is, if authenticated users are allowed to relay mail — then an attacker who subverts one user's account is then able to use the authenticated server as an open mail relay.
In such a configuration every user's password becomes a key to the mail system's security.
Spammers have attacked SMTP-AUTH mail servers by bruteforcing common usernames and passwords
A good password policy can effectively prevent such an attack

49. Acronyms

50. Acronym summary ESMTP - Extended SMTP ETRN - Extended Turn
IMAP - Internet Mail Access Protocol
MTA - Mail Transfer Agents
MUA - Mail User Agent
MX - Mail eXchange
POP - Post Office Protocol
SMTP - Simple Mail Transfer Protocol
UIDL - Unique IDentification Listing