Mail relaying and Exchange Server 5.5

Help — before the rest of my hair turns gray!

Recently I closed an open mail relay on our school’s Exchange Server. For the last few months, we have been using a portal administered by a local outfit. Until I closed the open mail relay, the portal enabled our faculty and students to read, reply and send e-mail through the portal to and from our Exchange Server. Now we can read the server’s mail, but not send or reply.
I added the portal’s IP address to the list of permitted routing hosts in the IMS config window, but it didn’t help. When I have been able to get the portal to route the mail, I also end up opening the mail relay again. So I am a little frustrated. Microsoft’s technical papers have not been helpful.