Relax postmaster checks #51

[bwalkerl]

Relaxed postmaster checks and added checking based on user email domains.

A couple of notes:

• The check on user domains is limited to the 5 most common for the mx lookup, but did we also want an additional check on gmail domains outside the top 5? If so should there be a minimum?
• Both tables in this check are rendered very roughly. Ideally they should use a proper template.

Closes #51