[dim dom]

So I went to my webmail account and added a 2nd email address but its not working. I was able to register with this EmailDiscussions forum using my first email address that was set up by my internet provider, but if I were to register using my 2nd email address I would not receive the activation email in my inbox. So I did a test and sent an email to myself(to my 2nd email address), the email did appear in the sent section but that email was sent to my inbox as Returned mail to my first email address. And this is what appeared in the email message- ---The following addresses had permanent fatal errors (shows my 2nd email address)
(reason: 550 5.1.1 <(shows my 2nd email address)>... User unknown)
(expanded from: (shows my 2nd email address)
---Transcript of session follows---
... while talking to [127.0.0.1]:
>>> DATA
<<< 550 5.1.1 <(shows my 2nd email address)>... User unknown
550 5.1.1 (shows my 2nd email address)... User unknown
<<< 503 5.0.0 Need RCPT (recipient)
QU- How do i fix this? Thanks.