Avoid localhost.localdomain in Message-ID of mails

When sending mails via CLI the message-id of the mails contains '<id>@localhost.localdomain'.
It would be much better to have the real domain in there.

We have to set this via the Hostname option of PHPMailer.
If this is not set, PHPMailer evaluates SERVER_NAME and then falls back to localhost.localdomain.
Merge pull request #24 from liayn/patch-1

Avoid localhost.localdomain in Message-ID of mails