Hi Colleen,
you could take a look at javascript to do this. Just take a short look at this discussion from a few days ago: https://community.marketo.com/MarketoDiscussionDetail?id=90650000000Ppy4AAC
This part that blocks mail domains which are not wanted:
var invalidDomains =
[
"hotmail.com",
"yahoo.com",
"aol.com",
"gmail.com",
"msn.com",
"comcast.net",
];
But, and this is just my opinion and experience, if people don't want to give their real email address they find a way (a lot of trashmail offerings exist...) so I prefer to build up data management campaigns which prevent your database to be flooded with this kind of information afterwards.
Hope that helps.