Send Notification Failed

Hi, I hope someone can help me?

I am having problems sending emails from SC+, the error is "Send notification Failed". The error in the logs is:

Caused by: javax.mail.SendFailedException: Sending failed;
nested exception is:
class javax.mail.AuthenticationFailedException

I have tried 2 different SMTP servers, both of which I have tested and confirmed working in Outlook, 1 required SMTP Auth, the other doesn't but neither work.

I have performed a packet capture during the process to see the SMTP server communication but it isn't logging any traffic on port 25?

All AV and Firewall software is disabled (as far as the eye can see!)

Does anybody have any ideas?