Strange Hotmail Problem

I have been having a problem for a few weeks now sending email to any
hotmail addresses. Any emails that I send to anyone with a hotmail email
address is not received by them. It does not bounce back to me and it
does not end up in their junk mail. It just disappears. I tried sending
mail to several different hotmail addresses with the same results. Mail
sent to any other email address goes through just fine. Any ideas what
is causing this problem? I have contacted MSN (no reply yet) and my
ISPwho suggested I contact MSN.

Sharon