We recently placed our servers behind Netscreen 5GT firewall to increase the security.
All web and mail servers behind the firewall has real (public) IPs and they were sending email to hotmail, gmail, yahoo etc. without any problem before getting behind the firewall.
However, after they started working behind firewall none of the servers can send email to hotmail accounts even though they can still send email to gmail, yahoo etc. and all other domains. Our Firewall IP is seen as our external IP now and it has a correct PTR record. It's IP is not listed in any of the spam blacklists. Our domains have SPF records.
Please send your thoughts about what could be the souce of this problem.
P.S. We also created a port forwarding on the firewall. If you telnet firewall's IP with port 25 it directs to one of the mail servers behind it.