Many people have been emailing support about this, so I felt it deserved its own announcement in case someone comes here looking for the answer.
Microsoft recently made changes to Internet Explorer that no longer allow a certain URL format.
http://user
domain.com:domain.com/ for example. This was because of an exploit many scammers were taking advantage of, and instead of fixing the problem, its easier to shut it off. There is no way to enable this format in your browser after you have updated as far as I know.
If you login to your Cpanel, then goto the Mail section and Add/Remove Accounts, there is a link "Read Webmail". This link will not work any longer for updated versions of Internet Explorer (Netscape seems to have this problem recently, too) because of the format of the link.
You now have only one way to access the webmail for your subaccounts. Use
http://yourdomain.com:2095/ Of course, please replace the yourdomain.com with your actual domain. Once there, make sure to enter the FULL email address as the login name, not just user, unless it is your main Cpanel account's email you are checking.
Also, if you want to skip the selection screen and go straight to a particular webmail client, the following links will work in the same way:
Neomail:
http://yourdomain.com:2095/neomail/neomail.pl
Horde:
http://yourdomain.com:2095/horde/index.php
SquirrelMail:
http://yourdomain.com:2095/3rdparty/...mail/index.php
Enjoy!