It is normal practice for organizations to register one or more "Mail Domains", and to have e-mail forwarded from those registered Domains to their dial-up account, thus giving them a more specific and descriptive e-mail address than provided by their dial-up account.
For example, typically a organization might have an dial-up account with the Internet name of "your-company.yourisp.com". This gives away the fact that they are using a dial-up account and does not look very professional. However, most ISP's and many independent organizations can arrange to register a domain such as "your-company.com" and arrange that e-mail destined for this domain is forwarded to your dial-up account.
If you already have e-mail forwarding set up with your ISP, you must
inform the Server system of this, otherwise it will reject the mail sent
to the Domain name(s) as the mail does not seem to be addressed to it.
Basically, it thinks the mail belongs to someone else.