Add support for tldBlackList

[vdust]

In the same way there is an option tldWhiteList, it would be nice to also have tldBlackList. A usecase for this is to forbid the use of user@localhost on production servers (which may have a valid MX record, so a dns check isn't enough), while still accepting any other user@tld that may be a valid public email address

[skeggse]

Sounds like a good idea to me. I can take a stab at this in a few hours. One comment, though - the localhost entry should be in the hosts file, and isemail sends an actual dns request (thus bypassing host caching and config), so you shouldn't be seeing this...

[vdust]

Actually, even though localhost does appear in the hosts file, the name server on the concerned host (which is the one set in resolv.conf) also has a zone file for localhost that contains an A record, but also an MX record. So the dns check does hit it and returns a valid MX record for localhost.

[skeggse]

I'm actually waiting on outmoded/hapi-contrib#62 before moving forward on this.

[hueniverse]

All done!

[skeggse]

@vdust keep in mind that a@[127.0.0.1] is valid for SMTP, so it might make sense to set errorLevel to 7 (dnsWarn) to avoid such an address being treated as valid.