Sending emails with KML attachment crashes mail service – re-setup required

Technical assistance
1

Hi everyone,

We’ve encountered a strange issue in our Zammad installation:

Whenever we send an email with a .kml attachment, the Zammad mail service crashes. After that, Zammad loses its connection to the mail server and fails to log in again.

Important:
Restarting the Zammad services (zammad-web, zammad-worker, zammad-scheduler) does not resolve the issue.
The only solution is to reconfigure the email account in the admin interface – after that, sending works again.

System information:

  • Zammad version: 6.5.0-1748004558.020e85a9.bookworm
  • OS: Debian 12
  • Mail server: Proxmox Mail Gateway (SMTP relay)

Observations:

  • The issue only occurs when .kml files are attached to an outgoing email.
  • After sending, Zammad permanently loses the ability to authenticate with the SMTP server.
  • Log files only show generic login/authentication failures.
  • The behavior is reproducible.

Questions:

  • Has anyone experienced similar issues?
  • Could this be related to MIME type handling or file content processing?
  • Any known workarounds or settings to prevent this?

Thanks in advance for any ideas or support!

2

Can you provide an example eml or kml or some logs about the crashing zammad services?

3

Hi everyone,

we were able to identify the cause of the issue.
The problem was not within Zammad itself but with the external SMTP server configuration.

In our case, the SMTP relay (Proxmox Mail Gateway) had a maximum message size limit that was smaller than the email size including the .kml attachment. When Zammad attempted to send the message, the SMTP server rejected it due to the size restriction. This caused Zammad’s mail connection to fail, and because of that rejection, it could no longer authenticate properly afterward.

After increasing the maximum allowed message size on the SMTP server, the issue no longer occurs — .kml attachments can now be sent without any problems, and the connection remains stable.

So if you encounter similar behavior, check the size limits on your mail relay or SMTP gateway first.

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