For debugging purpose or to perform IMAP actions on-behalf of an user, you can use pre-authenticated sessions. You can do this by simply running:
And you can start talking IMAP via stdin/stdout. This doesn't change process's UID, GID, or get any userdb settings. If you want to emulate a full regular login, you can execute
/usr/local/libexec/dovecot/imap -u username
Of course, you may need to run the above command as root if it needs to change process's uid/gid.