If you've upgraded from Fedora Core 1, it replaced obsoleted UW-IMAPd with Cyrus-IMAPd. To install Dovecot, first uninstall Cyrus-IMAPd:
yum remove cyrus-imapd*
Then install Dovecot:
yum install dovecot
Of course you can use too up2date or apt to do so.
If you migrate from UW-IMAPd and have mail stored inside the users' home directories as well, you then will have to do some steps foreach mail user:
cd $HOME # move all subscribed mailboxes to mail/ mv $(cat .mailboxlist) ./mail/ # move subscriptions file mv .mailboxlist ./mail/.subscriptions
For more explainations and details please see Migration.