On the first of January every year, I move the previous year's mails from INBOX, to 'all', which is an actual 'physical' folder in Mail.app - in other words, off the server and onto my Mac. I do this by selecting the last message of 31 December, and then scrolling to the 1 January of the previous year, and holding down SHIFT whilst I select that message. I then right-click on the selected mails, and choose 'move to' and 'all'.
In 2018, there are over 13,000 messages, so when I perform the above operation, I wait for a little while. They appear to have vanished from INBOX, which is good. I click on ‘Sent’, intending to do the same with 2018’s messages in there. But if I switch back to INBOX, hey presto – nothing has been moved. Worse, if I go and look in ‘all’, I see copies of the messages that I thought I had moved. If I did it ten times, there would be ten copies of those e-mails.
I know that there is also a ‘copy’ option on the contextual menu, but I do not click on that. I click on ‘move’.
Why are my messages not moving?