Is the message in the same account after the move or is the move simulating a deletion and an add? to get the message into another account.

Thanks for replying about my problem.

This involves moving messages within a single account (like from Notifications to Inbox). As I have repeated this process I have seen on a few occasions the destination (for example, Inbox) momentarily showing indications of the arrival of the message (Inbox shown in bold, message count area flickers) but immediately goes away. The message is not to be found in the targeted location/mailbox, or in any other location/mailbox for that account.

It is a recent problem, only surfacing this month (June. But I don't move stuff in mail much).

This problem does not occur on other machines (same version of Thunderbird, same mail account)

This happens when using the menu-based method of moving. Dragging and dropping seems to work (at least the ~6 times I tried).

I have repaired each mailbox in this account.

Is there a maximum size limitation for stable operation for Thunderbird mailboxes? I have about 900 Inbox messages, about 30,000 Newsletter messages, and about 20,000 Notification messages. The folders have been compressed.

I just tried the same operation again (move from Notifications to Inbox) and it was successful, but with 30 second delay. When the problem first occurred it happened repeatedly (more than 20 times, including after a system restart).

I continued to test by moving messages (menu method, and drag-and-drop), many times, and the moves are successful. Today all is well. I will re-test over a couple of days.

Exactly same antivirus program and settings? They can have very deleterious affects on IMAP moves as they are time dependent and antivirus is prone to being slow so things timeout.