Thunderbird fails to send multiple PDF attachments – Server timed out

Subject: Thunderbird fails to send multiple PDF attachments – Server timed out I am using Thunderbird on Linux Mint 21.3. Thunderbird sends ordinary emails without problems. However, when I attach two specific PDF files received by email from my bank, sending fails with the error message “Server timed out”. The message cannot even be saved as a draft. The files are very small (148 kB and 54 kB). I have established the following: Each PDF can be sent individually from Thunderbird after renaming it. Sending the two PDFs together fails, even after renaming them. Forwarding the original email containing the attachments also fails. Sending the same attachments together via GMX Webmail works immediately. Other emails sent through Thunderbird work normally. This appears to be a Thunderbird-specific problem involving multiple attachments or the processing of the original email, rather than file size or a general mail-server outage. Could you advise how to diagnose this issue and identify the cause? System: Linux Mint 21.3 (Virginia), 64-bit.