McAfee antivirus is among the top three troublemakers for Windows Mail.
Sometimes it takes a while before symptoms start showing up.
First of all, turn off email scanning in McAfee, although that is seldom sufficient.
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:
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://get.live.com/wlmail/overview
Even with WLM, you must disable email scanning in the antivirus,
for the reason explained here:
Gary VanderMolen, MS-MVP (Mail)