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Email basics

Wed 28 Nov 2018

Historical influences

Email was originally based very much on the office memorandum, which is structured with fields like so:

  • To:
  • From:
  • Date:
  • Subject:
  • Content (DATA)

Email servers and clients

  1. Store and forward
    • messages are stored and forwarded when appropriate
  2. Email servers
    • accept, forward, deliver, store messages
  3. Email clients
    • connect to email servers to send messages or retrieve messages
  • MUA, mail user agent. From email client to:
  • MSA, mail submission agent. The SMTP server does a:
  • DNS server lookup. From DNS to:
  • MTA, mail transfer agent. The SMTP server to:
  • Remote server. The remote server:
  • MDA, mail deliver agent to remote mailbox.
  • MUA, mail user agent, reads email.

Email headers

See RFC 5322:

Some important headers:

Originator fields

These three could be the same or different:

  • from: the author of the message
  • sender: the person sending the message
  • reply-to: reply messages sent to this person

Destination address fields

  • to: the primary recipient
  • cc: (carbon copy), secondary recipients
  • bcc: (blind carbon copy), multiple uses

Identification fields

  • message-id: a unique identifer
  • in-reply-to: a unique follow up identifier

Informational fields (human readable)

  • subject:
  • comments:
  • keywords:

Important comment on "Re:", Latin for "In the matter of" -- this is application dependent and not required.

Message body / Data

  • ASCII, American Standard Code for Information Interchange: character encoding
  • MIME, Multipurpose Internet Mail Extensions
    • non-ASCII text
    • non-text
    • and more

MIME file (~/.mailcap) on my system:

image/*; eog '%s'
text/html; w3m -I %{charset} -T text/html; copiousoutput;
audio/*; cvlc '%s'
#application/pdf; evince '%s'
application/pdf; mupdf '%s'
application/force-download; evince '%s'
application/x-msword; soffice --nologo --writer '%s'

Email Security

Some issues:

  • Spam
  • Malware
  • Phishing
  • Spoofing
  • Privacy
    • lack of encryption
    • intermediate systems
    • storage issues on remote systems
    • metadata
linux/email.txt · Last modified: 2019/01/21 11:27 by seanburns