Since you are using pine, I will assume you are using unix. Almost all *new* installation included the latest greatest sendmail with a certain support macros compiled in. One of these supports the virtusertable (usually located as /etc/mail/virtusertable). They keep changing around the syntax, but it will be one of the following:
Make certain that if their are other entries such as "firstname.lastname@example.org" or "email@example.com" that you place the entry after all the entries for mydomain.com or you will have irratic results (and in some cases, somebody will just get a LOT of mail).
Once you have made the entries, depending on your OS's setup (and the age of your installation) you will either just do a
or something a little uglier, which I wont post here and confuse you unless the 'make' comes back with an error. Post here if it does (along with the error) and I'll give the ugly version using makemap.
Of note, sometimes this same thing can be accomplished in your /etc/aliases file. Be certain to run newaliases after any changes to your alias file.