Mail is stuck on moving messages
It happened to me several times, sometimes the problem goes away, but sometimes not. After reading hundreds of messages I found that it happens when people in Mail only with Exchange servers, only when moving messages or deleting messages. Mail apps on phones and iPad continues to work, as well as mailbox online. But any changes done in Mail on Mac are not happening on the server and the number of moving messages continues to increase with every operation with mail you are trying to do.
Closing mail, rebooting, starting in safe mode, renaming mail folders is not helping. The only solution which seems to work is to delete some technical files in the Mail folders, which causes the app to re-download the entire mail collection! I did this once and despite it worked it is very stressful operation because no one wants to loose all the mail. Also, it takes hours to complete and I still don't know if the old mail database remained on the hard drive and takes place?
Long story short, my Mail stuck again two days ago. Number of mails in the message "moving 1 out of XX" was increasing and I needed to do something. What worked for me is an advise I found in previous threads: right click on Trash bin and "Erase Deleted Items". Just in case I did it for the Junk folder, even if it was empty. Then I took the account offline and back online. The message "moving messages" disappeared, the mail downloaded some messages as unread, some of messages I deleted on the Mac remained on the server, so I deleted using my phone app and it seems working now... Till it will stuck one day again.