User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Firefox/91.0 Waterfox/91.3.1

Steps to reproduce:

Create whatever forward rule you like under Tools - Message Filters.
(using Windows 10)

Actual results:

All email messages with an attachment are not forwarded, they fail with an error message that can be seen in supplied screenshot.

Expected results:

Email messages with an attachment should be forwarded without failing.

Thunderbird 91.3.2 (64-bits) on a macOS Catalina 10.15.7 (19H1519)

I'm experiencing the same problem. Same error message.
All my email forward filters with attachment doesn't work.
Forwarding without filter does work.

Hi,
same Problem here. With Thunderbird 91.3.2 and Windows 10 Pro (version 21 h1):
Automatic forwarding works without attachment, but fails with attachment!

This doesn't only happen when filters are in use.
If I send an e-mail with an attachment using a distribution list of 100, it will normally fail with the error message that has already been described after sending about 30.

Same Problem here, last observed 91.3.2 (64 bit) today. OS : Windows 10 pro 21H2, Mail server in: dovecot (IMAP) with plain auth on linux, out: postfix on linux (no auth required for internal ips)

worked with 68.x before.
Installed plugin that might affect it : lookout 4.3 (for TNEF winmail.dat decoding) - but that was active in working 68.x also without problems

I have the impression that it is working sometimes, but mostly fails. If I forward the mail manually, it always works

Further investigation show this probably is the same problem as bug 1741805...