Mail messages disappearing
After decades of reliable use, I am tearing my hair out over Mail. It has taken to simply "disappearing" messages I receive or send, without a trace. Sometimes they show up if I search for them, other times not. I can send a message, have it acknowledged as sent, and have it vanish without a trace or record. This is true for both accounts I use: a gmail POP account, and an Outlook IMAP account. Some messages in other saved folders vanish, others stick around just fine, apparently arbitrarily. I have tried rebuilding the mailboxes (I have yet to see that do anything, and wonder if that function is actually still operational), tried deleting the envelope files from the library (more messages go! none come back!), pretty much any ideas I've been able to garner from other threads. I am at the limit of being able to use it as a Mail client if I don't figure out how to fix it. Any suggestions welcome! Thanks in advance.
MacBook Pro with Retina display, macOS Sierra (10.12.5)