TLB, check this setting:
Go to Tools, Accounts, select that mail account, Properties, Servers.
Uncheck the box for "Log on using Secure Password Authentication."
If that was already unchecked, the only remaining culprit is your Norton
Antivirus. Norton security products are among the top three troublemakers
for Windows Mail. Symptoms often don't surface until several months have
gone by, or until an update occurs.
First, be sure to disable its email scanning, for the reason explained here:
http://thundercloud.net/infoave/tutorials/email-scanning/index.htm
Disabling the Norton program is often insufficient. Do a trial uninstall,
followed by using their debris removal tool:
http://service1.symantec.com/Support/tsgeninfo.nsf/docid/2005033108162039
Instead of uninstalling your antivirus, another option is to upgrade to
Windows Live Mail, which is more resistant to the adverse effects of overly
intrusive antivirus products:
http://download.live.com/wlmail