I'm assuming your form works just fine with non-AOL addresses?
When you said changing the "from" to the domain email worked temporarily, it makes me wonder if AOL isn't the problem. Have you checked to make sure your IP isn't on an AOL spam blacklist? You can check that here.
Using SMTP is a good idea, too. It will serve to make your emails look less spammer-like and be more reliable.
The other thing I would recommend--and I realize how difficult it is to pry clients free of AOL--would be to have them step up to an email address based on their domain. It looks more professional, advertises their "brand" with each email they send using it, and you'll have more control over the server's handling of the email. You also won't have to worry about trying get un-blacklisted with AOL if that turns out to be the issue.