Who filters the mail?

We have two ISP’s and are having trouble getting email from one to the other. Mail sent from zetes.host4u.com to uf.znet.com returns a “no route to host” error. We have had no trouble reaching the znet server with a traceroute from the host4u server, but email does not go through. Sending mail the opposite direction has no problem at all.

I have talked to support at both ISP’s and they say “well it’s not us”. Is it possible that there is a blackhole somewhere else along the route? Otherthan switching ISP’s is there something else I can do?

Thanks for your reply,
Glen
BAIE.COM