Been serveral years since using redhat 7.3.
Where is the mail for each user located?
Does /etc/dovecot file know the location?
Does it allow plain pop3 aceess (/etc/dovecot)?
No firewall in the way?
May have to create a tcpwrapper for this in /etc/inetd.conf. Been a long time since using that.
Really should be simply as installing and starting up. Have you check the docmention page at www.dovecot.org
? Might try contacting the developers for help as well.