> I didn't receive any mails sent via mailResume()
Ah, okay, we're getting closer :)
So, if your questionnaire is not yet online, you can either start it for testing in "compose questionnaire" or by using a pretest link.
If your respondents start via an iinvitation from a mailing, then you can set the debugging checkbox for an address entry, and test with that one.
So, please explain - as concise as possible - what you're trying to to and at which point you're stuck?