You say this happens to messages that you sent. I assume that you then mean that it is the recipient who can't access them, rather than you.
They need to change the settings in their e-mail client. If it is OE, then:
Click Tools/Options
On the Security tab, uncheck the box that says "Do not allow attachments to be saved or opened that could potentially be a virus"
Some people assume the line above indicates that OE checks in some way to see if an attachment is infected, but really all it means is that ANY attachment that MIGHT under ANY CONCEIVABLE CIRCUMSTANCES contain a virus, is blocked.