1. SMTP
SMTP stands for Simple Mail Transfer Protocol. SMTP is used when is delivered from an client, such as Outlook Express, to an server or when is delivered from one server to another. SMTP uses port 25.

2. POP
In computing, the Post Office Protocol (POP) is an application-layer Internet standard protocol used by local clients to retrieve e- mail from a remote server over a TCP/IP connection.
The POP3 service performs the tasks of message download and request handling on a Windows-based server, where message download consists of transmitting the messages from a folder in the file system to clients and request handling is performed according to the POP3 protocol, which defines how the server responds to requests sent from an client.

3. Advantages & Disadvantages of POP
Since all mails resides on your PC, you can read your mail without being logged onto the Internet.
There is no advertising when you read your unless you are using an Adware reader like Eudora freeware.
Opening attachments is a quick and painless process because they are already on your PC.
There are often no size limits on the you send or receive.
There is not a maximum size on your mailbox, except as determined by the size of your hard drive.

4. Disadvantages of POP Disadvantages
Opening attachments is a quick and painless process, unless the attachment has a virus payload in it.
If you have JavaScript enabled in your reader, you might be target of rogue JavaScript embedded in an .
All messages are stored on your hard drive eating up what is sometimes very valuable space.
folders can become corrupted and sometimes lost forever. Recovering is often a painful exercise.

5. IMAP
IMAP stands for Internet Message Access Protocol. IMAP shares many similar features with POP3. It, too, is a protocol that an client can use to download from an server. However, IMAP includes many more features than POP3. The IMAP protocol is designed to let users keep their on the server. IMAP requires more disk space on the server and more CPU resources than POP3, as all s are stored on the server. IMAP normally uses port 143. Here is more information about IMAP.

6. Advantages & Disadvantages of IMAP
IMAP has the following advantages:
Mail stored on remote server, i.e. accessible from multiple different locations.
Internet connection needed to access mail.
Faster overview as only headers are downloaded until content is explicitly requested.
Mail is automatically backed up if server is managed properly.
Disadvantages
Message storage may be limited. Reading messages while offline requires use of your program's Offline mode.
Requires Internet Connection to access the from IMAP