Dear Frank
Please check some configuration:
1- check SSL configuration, use below cypher, enable all TLS versions and disable all SSL versions:
https://www.axigen.com/documentation/a-grade-ssl-listeners-p3277035
2- check SPF configuration or disable it for a while.
3- check the size of email before send:
4- check all user and domain rules or anti-spam filters that sending domain does not contain it:
and finally I strongly recommend you to update Axigen to newer version.
Please send me feedback.