md5 hashing emails - that's dumb, right?

I work with nonprofits who often share their email lists lower cased and hashed with unsalted md5 in order to verify how many people signed a petition, etc.

That sounds like it would be fairly trivial to crack.  I haven't been able to figure it out yet.  But i'd like to show on my list rather than just say that it can be done.

Two Questions:

1.  Someone who knows what they're doing could unhash someone's email list fairly easily, right?

2.  What is the mask I should be using?  Roughly ten percent of my list has a dot in the name.  If they have a number in the name, it's at the end usually.

The vast majority of emails are from these domains.


@gmail.com
@yahoo.com
@aol.com
@hotmail.com
@comcast.net
@sbcglobal.net
@msn.com
@verizon.net
@earthlink.net
@att.net


I feel like I need to relearn regular expressions in backwards land....