The message is deleted from the folder itself but is not moved to the Trash folder. If you use the standard message store type "File per folder (mbox)" the message is not deleted from the correspondent mailbox file but only marked as deleted with the X-Mozilla Status: 0009

From - Thu Jan 29 19:01:52 2026 X-Mozilla-Status: 0009 X-Mozilla-Status2: 00000000 X-Mozilla-Keys: X-Account-Key: account1

So theoretically it can be restored, either by editing the mbox file in a text editor and changing the X-Mozilla Status from 0009 to 0000, or more easily with the extension "Undelete" https://www.ggbs.de/extensions/Undelete.html Yet as soon the original folder has been compacted - manually or automatically - the message will be wiped out from the mbox file and is definitively gone https://support.mozilla.org/en-US/kb/compacting-folders

Great information, thank you. I would prefer that the message go to Trash which would better fit with how I handle the "aging" of messages. It appears that is not currently a Thunderbird option, I wonder if that would be a major change? Again, thank you for the thorough and timely response!