Question : Email forwarding with 123-reg.co.uk and Exchange Server 2003

I am testing out email forwarding through 123-reg.co.uk. I have two domain names registered where I'm trying to forward user@domainA to user@domainB. I'm using Exchange 2003 Server and have set-up a single mailbox and listed both domainA and domainB in the emails tab in the user set-up. If I send an email directly to user@domainB this works fine but a forwarded email does not work - is this related to spam filters or something like that? How can I solve this problem?

Answer : Email forwarding with 123-reg.co.uk and Exchange Server 2003

excadmin; thanks for your input - the bounce was coming from my ISP - both domains are hosted at the same place and the MX records are identical. Once I changed the MX records on the destination domain all was Ok.
Thanks for keeping an eye on the posts!!