Migrating from postfix to postfix

[Hair]

Hello, the company I work for has decided to host web and mail internally
instead of paying a hosting company.  I have gotten freebsd set up and
postfix and squirrelmail up and running.  Is there a way to migrate saved
messages from the old server to the new one?  I tried simply stopping
postfix on both servers, copy /var/mail/username and restart postfix, but
the copied mail does not show up.  Thanks.

--Tommy Vielkanowitz