In this below link we can find compatibility issues
Since version 78.* Thunderbird uses new MailExtensions format, legacy extensions are no longer supported, most of released version before late 2020 becomes dead with newer version.
Here is how the New MailExtensions format looks like
Sophos E-mail appliances and SPX encryption.
If your e-mail passes through a Sophos device configured for SPX encryption it will see the header and encrypt the e-mail as it is configured.
Hence I request you to do needful with upgrading in newer version of Thunderbird 78.*
Hi NISHANTH J,
Thank you for reaching out to the Community!
Did you mean that updating the Thunderbird version to 78* SPX on the email appliance no longer works?
Did you open a support case? If yes, please share your case number by sending a personal message.