Message flows

This document explains at a high level some parts of chasquid's message processing, in particular how messages flow through the system.

Message reception

• Client connects to chasquid on the smtp or submission ports, and issues HELO/EHLO.
• Client optionally performs STARTTLS.
• Client optionally performs AUTH.
  • Check that this is done over TLS.
• Client sends MAIL FROM.
  • Check SPF.
  • Check connection security level.
• Client sends one or more RCPT TO.
  • If the destination is remote, then the user must have authenticated.
  • If the destination is local, check that the user exists.
• Client sends DATA.
• Client sends actual data, and ends it with '.'
  • Run the post-data hook. If the hook fails, return an error.
  • Parse the data contents to perform loop detection.
  • Add the required headers (Received, SPF results, post-data hook output).
  • Put it in the queue and reply success.

Queue processing

Before accepting a message:

• Create a (pseudo) random internal ID for it.
• For each recipient, use the alias database to expand it, add the results to the list of final recipients (which may not be email).
• Save the resulting envelope (with the final recipients) to disk.

Queue processing runs asynchronously, there's a goroutine for each message which does, in a loop:

• For each recipient which we have not delivered yet:
  • Attempt delivery.
  • Write to disk the results.
• If there are mails still pending, wait for some time (incrementally).
• When all the recipients have completed delivery, or enough time has passed:
  • If all were successful, remove from the queue.
  • If some failed, send a delivery status notification back to the sender.