Docker / Postfix Problem

We use Docker and Postfix to host a variety of websites. We have migrated to a new server, and it is giving us a hard time getting email to send from a specific docker container. All other docker containers email from postfix fine - except this one, which is a basic PHP / Lamp site. I can not seem to figure it out, and I have been working on it several days. The exact same container worked to send email on our old server for years. I need someone more versed in Linux servers, Postfix, and Docker to assist.