Hi,
I have this problem. I always see an error in the Status-mailing. When I send the test the message, it comes fine but I still get an error: Email sent with Newsletter: FAILED ().
You get failed but the email is delivered? Failed comes from the mail function on wordpress, it's strange to have that result if the email is actually delivered.
No, I'm not using any SMTP plugin (at least I don't know of any). The Mailing status shows KO (A test has never run). When I enter an email address (I tried several different addresses), I get this Failed error but I always receive the test email. And the status still shows KO - Test never run.
Thanks