cyrusimap
The latest stable/current release of the cyrus-imapd server is 2.3.16 which includes support for replicated mailboxes, unified murder configuration, delayed expunge, separate metadata partitions, Sieve extensions, and much more. It requires SASLv2. For specifics about the changes refer to http://cyrusimap.web.cmu.edu/imapd/changes.html. If you are using SQL detection, some changes may be required (http://cyrusimap.web.cmu.edu/imapd/install-upgrade.html).
dovecot
dovecot has been upgraded to 1.2.11. In earlier versions, some very large headers were sent which could result in a denial of service. This update fixes that problem, in addition to some security improvements. Details can be found at http://dovecot.org/doc/NEWS. (Note that Fefora 12 included version 1.2.6).
fetchmail
Fedora 13 includes version 6.3.14 of fetchmail. This update fixes some security related bugs and restores IMAP2 support for some servers. Details of the changes can be found at http://developer.berlios.de/project/shownotes.php?group_id=1824&release_id=17213.
sendmail
sendmail has been updated to 8.14.4. There are a number of bug fixes, including some security improvements.
Zarafa
zarafa is replacement for Microsoft Outlook, which is new to Fedora. It provides an integration with existing Linux mail servers and a web access with 'Look & Feel' similar to Outlook using Ajax. Zarafa includes an IMAP4 and a POP3 gateway as well as an iCal/CalDAV gateway, and can combine the usability with the stability and flexibility of a Linux server.