Block all SMTP traffic that doesn't go to MailHog

[pattifix]

All mail traffic should be blocked, that doesn't go to mailhog.
reasons for example:

• you get mails like "Undelieverable" mail as a shopowner even without setting up the mailer
• people might insert their microsoft users and passwords to get mails working (no one should do this anyway, but as this would also no longer work no one would do it)