If you are sending graphics or large attachments with the emails, you might try a longer pause.
I am not sending any attachments. I am now leaving the Name field blank in the smtp settings.
As a work around if I get the 1506 error, I place it back in the que for the server to send it again later.
I've read some of the other threads dealing with the irritating 1506 error, but this one best fits the problem I'm seeing…
Basically I'm sending emails out on one-by-one test basis and I'm getting mixed results with no discernible pattern. Some emails go out with no problems, but some get the 1506 error and STILL go out.
I'm leaving the From: name blank, which has been suggested before. I am sending attachments. I'm using the latest FileMaker Pro Advanced 11.0v2.
I'd use the queue idea, but then I'd risk sending out two or more identical emails because the 1506 error is unreliable!