Can email be shared between POP3 and shell account access?

I needed to receive a message to admin@mydomain.com. I waited and waited for it, but it never showed up in the admin mailbox on the mydomain.com machine. So I looked for the admin account in /etc/dovecot.passwd, and there it was, plain-text password and all! I set up a retrieval system for the admin POP3 account and got all the messages I was waiting for, not to mention about 20 that I wasn't, which were obviously old and discardable. This got me to thinking, is it possible to have an account accessible not just via POP3 from the ouside, but through mail/mailx on the inside, and if so, how is that set up?