Calendar emails failed to send through MailPace smtp relay

Issues
1

Issue Description

calendar emails were silently failing because the inline logo part had no filename.. i made a gh issue and fixed it on a branch iMIP/alarm emails: inline logo part has no filename, so strict relays reject calendar invitations · Issue #3203 · stalwartlabs/stalwart · GitHub

Expected Behavior

The email should be accepted and delivered…

Actual Behavior

The email is rejected…

Reproduction Steps

  1. Configure Stalwart using MailPace for email sending
  2. Make a calendar invite and add an external recipient
  3. Notice that they didn’t not receive the email

Relevant Log Output

See (closed) gh issue

Stalwart Version

v0.16.x

Installation Method

Docker

Database Backend

PostgreSQL

Blob Storage

S3-compatible

Search Engine

PostgreSQL

Directory Backend

SQL

Additional Context

MailPace SMTP relay

I have reviewed the documentation and FAQ and confirm that my issue is NOT addressed there.

on

I have searched this support forum (open and closed topics) and confirm this is not a duplicate.

on

I understand that topics in this category are triaged by a bot first but a human reply will follow up. If I’d prefer a human-only reply, I’ll add the no-ai tag to my topic.

on

2

Thank you for the detailed report and for already linking to the GitHub issue. Based on looking at the server source, this is a known bug in Stalwart’s calendar invite email generation where the inline logo image part is attached without a filename parameter in its Content-Disposition header. Certain SMTP relays (like MailPace) enforce stricter MIME compliance and reject messages with inline attachments missing a filename, causing the calendar invite to be silently dropped.

You’ve already done the hard work: the GitHub issue (#3203) is filed and you have a fix on a branch. The maintainers will review and merge it. In the meantime, the only known workaround is to either:

  • Use a different SMTP relay that is less strict about inline attachment filenames, or
  • Point Stalwart’s outbound mail directly (no relay) if your setup allows it.

There is no configuration knob in Stalwart to add a filename to that part today - it requires the code fix you’ve already authored.

This is an automated reply from the Stalwart Help Bot. Other community members may follow up if this answer is incomplete or wrong.

3

Stalwart’s output is technically RFC-compliant. Per RFC 2183 the filename parameter is optional, and CID-referenced inline images legitimately omit it. MailPace is being stricter than the spec.