not receiving emails

From: nomaildelivered (nomaildelivered_at_discussions.microsoft.com)
Date: 07/20/04 

Date: Mon, 19 Jul 2004 22:47:01 -0700

i have noticed that i do not receive all of my emails. One have mentioned that when he had sent me the email, that it bounced back to him and the email he had sent to me was undeliverable. I also have not been receiving other emails from reputable company sites. I have checked with my isp and they told me to send them(people/companies) an email in order to help resolve this issue, however, I do not feel that this is the case. I had even removed all blocked emails/cleared cookies from my list to see if this would even resolve the issue of not receiving my email, and still I do not get "very important emails" delivered to me. Please help.