Sent mailboxes and emails disappearing; different Sent boxes on different devices
After many years of happy use, my Sent mailboxes have started acting really weird lately.
I found that Sent messages were being saved in different mailboxes on my iPhone, MacBook and iMac. Not all sent messages were showing up in the catch-all "Sent" box that shows items from all accounts.
Tried to solve this but couldn't, so labelled the various mailboxes "Sent from phone", "Sent from laptop" etc. That seems to have caused even more problems. Now if I try to set a particular Sent mailbox in Accounts > Mailbox Behaviors, the mailbox I choose immediately disappears!
This has resulted in many sent emails also disappearing. Sometimes they show up in Recovered folders, but often they disappear for good.
Please help!
Apple Mail 11.5
iMac Line (2012 and Later)