I've been getting the Formbuilder "Mail server error. Unable to send email using the built-in WordPress mail controls" error lately (could be because I upgraded the wordpress version...?). However, the test emails sent through my wp smtp mail plugin and Subscribe2 plugin work just fine.
I've found this to be a problem with Fast Secure Contact Form too. The test email function sends beautifully while sending through the form results in no email.
Anyone have any idea what's up?