Error 324 on select responses

We've got a form which went live this morning, which is getting loads of entries and it's critical that it works correctly. The form is at http://www.moving-4-ward.com/register/register-for-the-industry-forum/

The form seems to have issues with certain email addresses. When I try and sign up with the following information, the form returns error 324 and there is no response from the server.

Name: Robert Nagel
Company: SNC Lavalin Transportation USA
Address: 13901 Sutton Park Drive South
Suite 200
Jacksonville, Florida 32224
Phone: 9044773765
Email: Robert.nagel@snclavalin.com

The entry is recorded in Gravity Forms, however, the email address is not forwarded to iContact (I'm using the iContact plugin: http://wordpress.org/extend/plugins/gravity-forms-icontact/ - I tried removing the feed from this form, but it didn't affect it). If I change the email address however, it goes through OK.

It is critical that these forms are up and running ASAP. Any ideas on what the issue might be?

This wasn't really resolved, because the answer (both my own and on the recommendation of Priority Support) was basically to move to a new server (from a GoDaddy VPS to a ServInt VPS), and the issue went away, but it's no longer an issue because the error isn't happening on ServInt.

The error was happening when the form was submitted. The entry was recorded in the database, but the user was not getting added to the iContact database, nor were the notification emails being sent.

There was no particular pattern to the errors: some users with periods in the email address were registering just fine, and some weren't getting through at all. I still don't know what the issue was, except that it was something to do with the server set up.