Thunderbird can't send messages longer than ~930 characters

Asked by Herbert van Kampen

Ubuntu: 20.04 LTS
Kernel: 5.13.0-40-generic
Thunderbird: 91.8.1 (64 bit)

Since starting service with an new ISP, I have been having problems with Thunderbird outgoing mail (incoming works fine). I did not see this problem with my previous ISP.

In both plain text and HTML formats, if the body of the email is greater than approx. 930 characters, when I click <Send>, the progress window goes to 100% sent, then hangs and eventually times out with the message not being sent. It shows an error message:

"Sending of message failed
The message could not be sent using Outgoing server for an unknown reason. Pleas verify that your Outgoing server settings are correct and try again."

I have contacted the new ISP about this and they have no idea what is causing this except it is not a problem with their SMTP host. All email settings are as directed by the new ISP.

I have confirmed this on 2 computers, both running the same software by creating messages with known character counts of over 1000 characters, created in a basic text editor, and progressively deleting characters until it is successfully sent and recounting the characters.

I do not see this problem on a Windows 10 machine using the same version of Thunderbird. I have also tried different email clients on Ubuntu and see the same issue.

I am at a loss, so any possible clues as to what is going on would be greatly appreciated.

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Bernard Stafford (bernard010) said :

Can you help with this problem?

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