I would look at two things:
1) Click on the Form tab and make sure that the email field has the right specification. It should look something like this:
<p><label for="email">Contact Email*:</label>
[email* your-email]</p>
2) Click on the mail tab, and enter an email address from your domain in the 'From' field. This solved most of my problems. To make sure you are able to reply to the sender you can add this entry in "Additional Headers": 'Reply-to: [name] <[your-email]>
Hopefully one of these will help. 90% of my submissions get through successfully but I'll leave my problems for another forum post:)