Looking at the test logs, it seems I inadvertently sent the test mail using the PHP mail() function and not via SMTP. So the error lies in my SMTP settings and not the form or plugin.
What I didn't realize, and may be cause for some confusion with other users, is that the test mail sends according to the send mail option settings. So (and this is what got me) it's not a pre-launch test per se, but a quick test area (rather than having to visit your form page and send from within). Helpful if you have validation, having to fill out added fields, etc. Sot the test area makes sense, just a little confusing. Well, for me anyway :)
Hope this helps others who may have misunderstood the test function.