I have two forms on a site, and both of them are behaving the same way.
When I use the email address I'd like to use (username@mydomain.com) in the "To:" field in the forms, I get a red bordered error saying "Failed to send your message. Please try later or contact the administrator by another method." This particular email address is a Google Apps address using the future domain name of the site (I haven't changed the DNS to point to the new host/test site yet). This is a working email address.
When I change the "To:" field to my personal Gmail address in the form settings, it sends just fine. The forms are working perfectly.
If it matters, I'm hosted on BlueHost.