If `sendmail` does not exit, the `sendmail`-based mailers hang indefinitely and block the queue. We should put a timeout on `sendmail` execution.
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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.