And another Apple Mail Problem: An error occurred while moving messages to mailbox “(null)”.

Solution: trash was not assigned correctly by Apple Mail when setting up accounts again. In "mailbox behavior" I found out that the mailboxes assigned didn't really exist and I set them right - this stopped the annoying pop-up, and the trash landed where it was supposed to land: in the trash (and not on the floor, on the wall, under the table, or out the window )

I think it is simply that Apple Mail is a bit a sloppy.

If I were to write a hit parade of annoying pop-ups that pop us when something doesn't work (and which are not helpful), the one that caused this thread would be in the top 5.

I browsed the web for this Apple Mail bug - none of the solutions worked.

When I click "delete" I get that An error occurred while moving messages to mailbox “(null)” message and the message disappears.

But where?

I does not show up in the trash mailbox.

Thanks, Apple, for stealing another hour and a half of my work day with your sloppy coding.

I deleted the account and set it up again.

This is a serious bug - the problem persists.

Upon clicking "delete" I get the error message, have to click "OK" even though it's not OK, and the message disappears into the void - never arrives in trash.

mailboxes are set up that trash goes to "deleted messages"