Is there a way to mass message more people via mail? Or another way to contact specific people?

More then 20? Mail is limited to 20 right now

If it helps, I know you can search through your friends with a clan name.

But that's for invites to games; I don't know how to use that for Mail (which is obviously not that user-friendly, unless I'm using it wrong).

Mulis script allowes you to search your friends list when making a mail :D

Well, without Muli's script, you can always Control + F to use the browser's search function. It's slightly laggy, you'll have a terrible time finding people with cloned names (e.g. "death", or all these "adolf hitler"s around) and some names are just flat-out unsearchable (*cough* juq's *cough*).

But I suppose most of these also are troublesome with the script :P

Anyway, yeah, you can mass mail, but only 21 players in a mail thread at a time.

Why would you want to blacklist a whole clan?