Problem

POP3/IMAP email clients reports problems downloading messages Windows Vista.
The mail client doesn't receive anything past the RETR command and the session times out after a while.

Solution

The solution is described in the following MS KB article:

http://support.microsoft.com/kb/934430/en-us

After applying the fix suggested by the article mentioned above, everything starts working again.

Important: the netsh command must be run explicitly with administrator privileges, even if the current user already owns them.