Mail problem

Hey there,

I've recently set up a new POP account. Now it was working fine but now every time I try to send the server changes from mail.madasafish.com (which works) to mail.madasafish.com:markpaterson (which won't send).

I've adjusted the outgoing server to mail.madasafish.com but it keeps telling me that it can't send from mail.madasafish.com:markpaterson

Anyone know what's going on? Have I explained this properly?

Thanks

Shark