Default HELO hostname in email notifier.

See https://github.com/mikel/mail/issues/640 for further details.

Requiring the hostname to be specified when talking to local MTA
is silly.

Lastly, the correct term is HELO hostname, not domain.
Merge pull request #258 from p-push/email-notifier-default-helo-hostname

Default HELO hostname in email notifier.