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E-mail System OPS335 R. Chan
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Email: concepts & protocols
Mail Systems: User agents, submission agents, Transport agents, Local Delivery agents, Message stores, Access agents Protocols: SMTP (ESMTP) POP3 (POP3S) IMAP (IMAPS) more ...
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Email system componets
UA: user agent DA: delivery agent MSA: submission Agent AA: access agent
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UA: User Agent A.k.a Mail User Agent or email client: Read mail
Compose mail Supports: Plain text MIME – multipurpose Internet Mail Extensions e.g. /bin/mail (mailx)
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Submission Agent Sits between UA and MTA Uses port 587 Speaks SMTP
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Trsansport Agents Speak SMTP (now also ESMTP) See RFC 821 ... RFC 5321
Take incoming Chek recipents' addresses Rewrite recipent/sender addresses if needed Forward message to next MTA/local DA e.g. Sendmail, Postfix, MS Exchange
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Local Delivery Agent Accepts mail from MTA
Delivers to recipients' mailboxes Notes: mail recipient could be A person A mailing list A program
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Message store Mbox (simple – a single file)
Maildir (supports mail folders) Location and access premission /usr/spool/mail Read/write by owner
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Access Agent Use to access message in the message store
Use POP3 or IMAP to communicate with the message store server Secure version: POP3S and IMAPS Difference between POP3 and IMAP e.g. outlook, thunderbird POP3/IMAP servers: Courier, Cyrus IMAP, Dovecot, Zimbra
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Componets of a Mail Message
The Envelope The Headers The body of the message Examples
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SMTP Simple Mail Transport Protocol
A clear text command/response protocol Key commands HELO, EHLO MAIL FROM: RCPT TO: DATA QUIT
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A Smaple SMTP Session See the Demonstration
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MTA Configuration Postfix
/etc/postfix/main.cf (see lab for basic settings) Commands: postfix postcat postconf postsuper
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and DNS Domain name MX record Mail routing relay
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MTA Debugging Postfix Mail Log entries - /var/log/maillog
Syslog - /var/log/messages Postfix directory /var/spool/postfix telnet [MTA IP] 25
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Questions & Answers ??? ? ? ? ? ? ? ?
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