1506 error, SMTP randomly does not send emails
I have seen this issue on the forums without answers - some a couple of years old.
We have several Mac and PC users sending emails via "Send Mail" command in a few scripts. A couple of years ago one person started having problems sending, it seemed random. I set her back on the SMTPit plug-in. A month or two ago, we have had 3 more users randomly experiencing the problem. Sometimes the emails send, sometimes not. The attachment does not seem to affect this, as some of the errors come from emails without attachments, and some come from emails that have attachements. The "To" field only contains the email address. Our IS person said nothing has changed with the SMTP server and all the settings are correct. The only error we get is 1506 so we can't tell what is happening.
I had problems like this and just gave up eventually. I created a googlemail account and used their SMTP server. I just do that every time now, and I have never had a problem.