Your biggest problem is McAfee. Don't allow McAfee to mess with
your email program because it will royally screw it up [sorry for the
technical terminology].
McAfee security programs are among the top three troublemakers
for Windows Mail.
First of all, disable all interaction your McAfee program has with email,
although that is seldom sufficient.
Next, delete that email account (because it has been corrupted by McAfee),
restart Windows Mail, then recreate the account.
If the problem continues, the only way you can prove or disprove that
McAfee is the culprit is to uninstall it, followed by using their debris removal tool:
http://service.mcafee.com/FAQDocument.aspx?id=TS100507
If you decide to replace McAfee, we can recommend a free antivirus
program which is more compatible with Windows Mail:
Instead of switching to a different antivirus, another option is to upgrade
to Windows Live Mail (WLM), which is more resistant to the adverse effects
of overly intrusive antivirus products:
http://download.live.com/wlmail
Even with WLM, you must disable email scanning in the antivirus,
for the reason explained here:
http://thundercloud.net/infoave/tutorials/email-scanning/index.htm