E-mail

Components

The Internet mail system has three major elements:

1. MUA (mail user agents)
   • allow users to read, reply to, forward, save, and compose messages
2. MTA (mail transfer agents) or mail servers
   • core of the e-mail infrastructure
3. protocols (e.g., SMTP for sending, IMAP/POP3/HTTP for retrieval)
   • SMTP uses TCP for reliable delivery between mail servers
   • SMTP is a client-server protocol, each mail server implements both roles depending on direction:
     • when a mail server sends mail to other mail servers, it acts as an SMTP client
     • when a mail server receives mail from other mail servers, it acts as an SMTP server

SMTP

SMTP (RFC 5321) (Simple Mail Transfer Protocol):

• much older than HTTP
• legacy technology that possesses certain archaic characteristics:
  • restricts the body and headers to ASCII
  • requires binary multimedia data to be encoded to ASCII before being sent over SMTP
  • requires the corresponding ASCII message to be decoded back to binary after SMTP transport

SMTP uses a store-and-forward model, where messages may pass through multiple intermediate servers before reaching their destination.

If a destination server is temporarily unavailable, sending servers typically retry delivery for some period before returning an error.

Mail message formats

A mail message:

• is defined in RFC 5322
• consists of a header and a body separated by a blank line (CRLF)

Mail access protocols

Mail servers run on always-on computers managed by an e-mail provider.

E-mail delivery happens in two stages:

1. submission and transfer (push protocol):
   • a user agent sends the mail to a mail server using SMTP (or HTTP in webmail)
   • mail servers then relay the message using SMTP until delivery
2. retrieval (pull protocols):
   • users retrieve messages from their mailbox using:
     • IMAP (Internet Message Access Protocol): keeps mail on server, synchronized across devices
     • POP3 (Post Office Protocol v3): downloads mail to the client
     • HTTP: used by webmail services

Why is the relay system useful?

• if Bob's mail server is temporarily unavailable, Alice's mail server can keep retrying delivery automatically
• this makes e-mail more reliable than direct device-to-device communication

End-to-end email communication flow

Alice wants to send Bob a simple ASCII message:

Step	Text
1. Alice	opens her mail client (MUA) provides Bob's e-mail address (bob@someschool.edu) composes a message instructs the user agent to send the message
2. Alice's mail client (MUA)	sends the message to her outgoing mail server using SMTP where it is placed in a message-outgoing queue
3. SMTP process on Alice's mail server	retrieves the message from the queue looks up Bob's mail server using DNS MX records opens a TCP connection to the destination SMTP server (or an intermediate relay) sends Alice's message into the TCP connection after some initial SMTP handshaking
4. Intermediate mail servers (MTAs)	may store and forward the message through multiple hops each MTA receives the message via SMTP and relays it closer to Bob's mail server
5. Bob's mail server	receives the message via SMTP places the message in Bob's mailbox
6. Bob's mail client (MUA)	connects to Bob's mail server when Bob checks his mail at his convenience retrieves or synchronizes messages using IMAP (or downloads them using POP3 or HTTP) displays the message to Bob