Search Articles

SMTP #

• SMTP works over TCP
• The MTA (message transfer agent) actually sends the mail over TCP
  • The system administrator sets up the MTA
• The user-agent is what the user interacts with, and this communicates with the MTA to send the mail
  • This is the gui (e.g. outlook) or cli (mail)
• Each host has a mailbox
  • All SMTP is is a protocol from sending mail from one system to the mailbox on another host
• DNS mx records indicate mail servers for a given host
  • e.g. can run host google.com and some mail servers will be listed
    • host -a google.com will print out preference values for each alternate mail server
    • Mx records with the lowest preference value are tried first
• If your server has a domain name of mydomain.com, each user will have a mailbox of user@mydomain.com
  • (all system users don’t necessarily have a mailbox but can be added to)
  • The mailbox will be in the /var/mail/<user> directory
  • You need to install an MTA to send mail locally
    • Can see this by trying to run mail --debug-level=2 <recipient>
    • It will try to send mail from <user>@hostname.com

Email Parts #

1. Envelope (MAIL from, RCPT To, etc.)
2. Headers
   • Possible headers
     • Reply-To
     • X-Phone
     • X-Mailer
     • To
     • Subject
     • Message-Id
     • From
     • Date
     • Received
     • etc.
   • Headers starting with X are used-defined
   • The user-agent adds some headers, then the MTA adds some headers
3. Body

Relay Agents #

• Generally, local MTAs communicate with a relay MTA which then communicates with the internet
• Any nonlocal outgoing mail gets sent to the relay agent
  • The relay agent then passes it on to the destination
• SMTP is used for this connection between local MTA and nonlocal MTAs

General Timeline/Outline #

1. Client queues message through client user-agent
2. Message is passed to client MTA
3. Client MTA opens up TCP connection to the destination mail server (where the mail is going/mailboxes are hosted) and sends the mail
4. This goes into the user mailbox
5. The user then uses a user-agent to view the mail

Multipurpose Internet Mail Extensions (MIME) #

• SMTP supports extensions that are backwards compatible with the original protocol
• Originally, email body had to be NVT Ascii
• MIME defines an extension that allows for structure in the email body
  • It just adds new headers that tells the recipient the structure of the body
• MIME Headers (not all required)
  • Mime-Version
  • Content-Type (e.g. Text/Plain, Application/JSON)
    • The first part is the content-type (e.g. Text) and second the subtype (e.g. Plain)
    • The multipart MIME Content-Type supports a body with multiple content types split up throughout the message
  • Content-Transfer-Encoding
  • Content-ID
  • Content-Description
• You see these headers describing HTTP request bodies as well
• MIME allows for arbitrary binary data to be exchanged
• MIME has now become a generic way of describing the structure of a document, file, or assortment of bytes. Thus it is used in HTTP requests
  • Though they are sometimes referred to as media types within the web