Is it really the user name that is the problem? I have worked with 550 before and it has always been a domain issue.
550 can mean anything the recipient site wants - it just means a permanent rejection.
I've already told GlitchFreak how to change the sender address, so I don't understand why we are asked whether it is possible. In any case, it is a bounce message which is not being accepted, not mail with sender address of root@ or admin@.
GlitchFreak needs to do more homework, or hire a competent consultant.