Have you set the Username and Password to that of your mail provider, and not for the NAS? That is what your mail provider will expect. You might also need to check that the email address you are using as the sender's email address, will authenticate through your email provider.
For example, if your normal email is joe.bloggs@myprovider.com, then your mail provider will only accept that with the associated Username and Password combination. You won't be able to use dns.345@myprovider.com for instance, as it will be an invalid address for the Username/Password combination. If you're running your own mail server, then the email address can be anything you like, as long as your mail server is given a correct set of Username and Password details.
If that's the case, then the sender address and the recipient address might need to be the same.
Edit: I have just tried testing this through my own mail server, but (and I am not shocked to say this), it only allows a Username that is a maximum of 31 characters long. My domain name is 21 characters long on its own. It must be the only device I have ever seen, that imposes a character limit on email addresses. [insert facepalm smiley here]