Implement a stub MailSubmissionService.

Send SMTP traffic to it so far. Abandon the HTTP aspect of the
connection.
Join all const declarations into one.

Looks slightly cleaner.
Send results to SpamAssassin and record the result.

As a bit of a regression, we have to stop keeping the mailstream
connection around forever, since it’s going to fail. We need a retry
mechanism here lateron.

So far, we still don’t do anything with the mail.