Unable to send email to specific people

Since June 1, 2005, I have been unable to send email to ONE particular
aol.com email address as well as ONE particular yahoo.com address. I
know I have the correct address and I cannot even reply to an email
sent to me by either of these people. I have been in contact with my
ISP at least 6 times and they tell me it is on my side. I have signed
in as root, and the problem is still there. I have uninstalled and
reinstalled Entourage without luck. Other than totally erasing my hard
drive and starting over again, I have no more ideas. Oh, yes, I can
send email from a different computer using my same setup except using
Entourage X instead of Entourage 2004. I am using OSX.3.9 on a G4, with
768 MB of memory and an 80 GB hard drive. I installed Entourage 2004
around the first of April. Suggestions?