Have you seen https://support.mozilla.org/en-US/kb/cannot-send-messages ?

This help page referred to TB v,78 nothing about v,115? followed all the all the solutions but to no avail. I believe this problem has appeared with other users since August 2023. Contacted technical service "Geeks2u" and they couldn't help and believe this version still in "Beta" phase. Found it strange that TB 115 release notes state "Thunderbird version 115 is only offered as direct download from Thunderbird.net and not as an upgrade from Thunderbird version 102 or earlier. A future release will provide updates from earlier versions." yet it downloaded automatically as an upgrade. Still confused that I cannot re install TB v.102 and no one has provided me with an answer.

To refresh my problem, each time I send message the following windows appear- 1) "unable to authenticate outgoing server(smtp)" . 2) " Please check password and verify "Authenticate method in Account setting Outgoing Server (smtp)". Not able to take a screen shot