Scans against port 143 (imap) are up considerably today:
http://isc.sans.org/port_details.php?port=143
This coincides with the release of an exploit against imap server in Mercury Mail 4.01 (aka Pegasus Mail). For details, seehttp://www.pmail.com/ . I don't think this package is very popular, but some Windows users may use it as an easy to administer/install mailserver.
In addition, a number of vulnerabilities against the popular Cyrus IMAP server where released last week:
http://security.e-matters.de/advisories/152004.html