OE is regarded as utterly obsolete by most techs due to the inbox limitations as SD indicated. I find that my OE is requesting to compact OE files more often than I'd like it to.
However, I find that deleting unnecessary emails (advertising, news, mates jokes, etc) every few days from the inbox, and creating additional folders occasionally to store emails you feel the need to keep, helps keep the inbox file size down.
At risk of repeating myself and sounding like an ad... by far the greatest gift to OE users, is Mailwasher (Mailwasher.net).
Mailwasher lets you preview all your incoming mail, delete what you don't want, or are suspicious of, and even bounce garbage back to spammers (very satisfying feeling, that...
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I'd suggest that your inbox is getting close to its file limit, and maybe Macafee is part of the problem. I ditched the everyday anti-virus programs such as Norton and others that are so invasive, and deathly to download speeds... and I now use Online Armor, which is far less invasive.