Had a client getting this error when trying to send emails last week and have now spent several hours on Google trying to find a solution.
Biggest catch is that other scripts were able to send mail, so it seemed unlikely that it was the host. So I took the phpMailer class and uploaded it to a test folder and wrote a script that did pretty much exactly what ProofBuddy does and it worked just like it should.
After much confusion on my part, I discovered a typo in the email address that was used for sending. A character was left out – exampl.com instead of example.com. Guess 1&1 had something in place that checked if it was a valid domain before sending out.
I’m sure there’s a lesson in there somewhere. Either way, I’m glad it wound up being something relatively simple even if it did take me way too long to get there.