But I just learnt that it could mean the opposite. So you can get this error, if the email connection is dropped or server doesn't answer quick enough after email was sent.
Now for some servers this means that checking attachment for viruses takes a second too long and FileMaker considers it as an error (timeout), but email is still sent.
So please don't use error 1506 as the truth about the email sent / not sent status. More like an "Email was most likely not sent" error, but you can't be sure.
Otherwise, if you use MBS Plugin with the SendMail functions, you could of course always check the debug logs from our CURL functions for exact status.
There you would see difference between timeouts and error from server.