We use the Add-On "Mail Merge" (https://addons.thunderbird.net/en-US/thunderbird/addon/mail-merge/?src=search) to create lists for Outgoing mail. It adds the individual messages to the Outbox and then when you "Send Unsent Mail", Thunderbird spools them one at a time. Noticed with a recent update to 144.0.1 this behavior hangs. It will send out maybe 5 messages and then just stick at "Sending Message" under the Activity Manager view. If you close Thunderbird and the Activity Manager and reopen, you can then do "Send Unsent Mail" and another 5ish will send before the behavior repeats. I attempted deleting the Global Database from Profile directory, rebuilding, uninstalling/reinstalling 144.0.1, eliminating AV scanning, turned off Indexing, emptied out directories, nothing seems to change the behavior. Sending individual messages works fine. Rolled back Thunderbird to the 140.4.0esr release and the program will send normally without the hang. Updated to 141.4, also works fine. Updated to 142.4, this is the first version I see the behavior. I collected a Performance Profile which can be reviewed at https://share.firefox.dev/4nxcltl for the issue happening on the 142.4 version.