If sendmail does not exit, the sendmail-based mailers hang indefinitely and block the queue. We should put a timeout on sendmail execution.
I've no idea why, but there always seems to be a bunch of PhabricatorMetaMTAWorker tasks that daemons pick up, that have large expiries and don't seem to go away in a short timeframe. I'd expect sending email to be a reasonably quick operation, and that these workers shouldn't just sit there for ages.
There's no exceptions in the daemon logs.