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E-mail, SMTP & IMAP

COMP211 Tutorials

There are three major components to e-mail:

  • User Agents
    • Such as outlook or geary.
  • Mail servers
    • Mailbox - contains incoming messages for user.
    • Mesage Queue - of outgoing messages.
    • SMTP Protocol - Between mail server to send email messages.
  • The protocol: simple mail transfer protocol (SMTP)

SMTP

The standard for this protocol is RFC 5321.

  • Uses TCP on port 25.
  • Direct transfer from the sending to receiving server.
  • Three phases of transfer:
    • Handshaking
    • Transer of Messages
    • Closure
  • Command/Response Interaction (like HTTP):
    • Commands: in ASCII.
    • Response: Status code and phrase.

Example SMTP Request

SMTP requests are sent as multiple objects, sent in a multi-part message. The message is split based on who is responding.

220 EventMachine SMTP Server

HELO alice

250 Ok EventMachine SMTP Server

MAIL FROM: <sender@example.com>

250 Ok

RCPT TO: <receiver@example.org>

250 Ok

DATA

354 Send it

SUBJECT: Hello

Hi Bob, How's the weather? Alice.
.

250 Message accepted

The message is ended with \r\n.\r\n. Where \r is a carriage return and \n is a newline.

Mail Message Format

SMTP is defined in RFC 531 (like HTTP). Additionally, RFC 822 defines the syntax for e-mail messages (like HTML).

An SMTP message can be split into two parts:

  • Header Lines:
    • To:
    • From:
    • Subject:
  • Body - Message with ASCII characters only.

Mail Access Protocols

SMTP can send mail but in order to retrieve it and read the contents you must use other protocols:

  • SMTP - Delivery and storage of e-mail.
  • IMAP (Internet mail access protocol RFC 3501) - Messages stored on server, provides retrieval, deletion & folders of stored messages.
  • HTTP - Provides web-based interfaces on top of SMTP & IMAP.