That is a possibility. Have you been through the smtp logs on the exchange server to see what it shows you as the reason for the disconnect, like session was dropped, or session was disconnected or session rejected? Try getting the exchange admin for the receiving company on the phone if possible. They may be using a third-party filter like we do or they my not, but they can help identify the problem. Our third-party filter company restircts by IP and sometimes we have to get that corrected.
Check your firewall rules. Be sure that port 25 outbound is permitted to all destination addresses unless you use a relay host. I have seen issues where one rule is applied to outbound traffic and a different rule to inbound traffic which means session state was lost. This effectively broke the 2-way conversation. You may need to do some packet snoops to capture exactly what is happening on both sides of the firewall to fully understand the issue.