Add dns lookup to check for email domain aliases.

[motionless]

This should help to make the blacklist more valuable.

Context:
Trash mailers provide email aliases that are mapped to a different domain. Using resolv to get to the original MX record helps to catch more email addresses that coming from trash mailer.

[cbhp]

Hi and thank you for your contribution. We haven't implemented DNS lookup so far because of speed loss and the big amount of queries. I have some questions:

• What happens if the record lookup times out?
• What's the default timeout?
• Why is the record lookup performed always, not only if the domain is not found in the blacklist?
• Is it possible to somehow cache the record lookups?
• You shouldn't just take the first MX record, but the one with lowest preference value
• You should also apply .downcase to the MX domain
• It's more likely the main_domain is in the blacklist than the complete_domain. That's why we had the checks in that order. Is there a reason why you changed the order?