Ive got an intermittent reoccurring issue on mac mail. I have this issue on 2 machines. Both machines are macbook pros, one is running 10.10.5 and the other 10.12.latest sub version.
The issue/s are that :
a) some emails that are sent will just sit in the outbox and not send, if i close down mac mail and re open it it will start sending these emails. This doesn't always happen but at least once a week it occurs.
b) an email will be received but wont show up in mac mail, its definitely received as it will show up when viewed online in gmail, and will show up on my phone (iOS mac mail) but wont show up on the OSX mac mail. Whats strange is that this wont happen to all emails, but a select few, eg. i might receive an email at 11am which doesn't show up on OSX mac mail, but at 11.30 and 11.45 two more emails may come through that i do get.
Each machine has access to some of the same accounts (but not identically the same)
Most of these accounts are google apps for business gmail accounts, but some are also IMAP accounts with my web host.
For the most part this seems to be happening with the google apps for business gmail accounts, because we use those more, but i think it also happens a bit for the web hosted IMAP accounts.