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, which must be done over TLS.
• Client sends MAIL FROM, and we check SPF and the 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 '.'

Then, to process the data:

• Parse the data contents to perform loop detection.
• If the sender is authenticated, DKIM-sign the email with the corresponding key.
• If the sender is not authenticated, verify the DKIM signature (if the email has one).
• Add the required headers (Received, SPF results, post-data hook output).
• Run the post-data hook. If the hook fails, return an error.
• Put it in the queue and reply success.

Authenticated mail, and email spoofing

By default, authenticated users can send emails as any other user or domain. For example, you can authenticate as a@a, and send email as b@b.

This is a design choice made to balance simplicity of operation and use.

Users who want to be strict about "MAIL FROM" or even "From:" validation can add additional checks in the post-DATA hook.

In the future, chasquid may get some option to be strict about it by default, or on a per-domain or per-user basis. But for now, using a post-DATA hook is the best way to make chasquid more strict about this.

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, and write the results to disk.
• 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.