Thanks for the insight, but this is the first time a situation like this has happened, the server has been operational since 2014. It is a private, low volume

Thanks for the insight, but this is the first time a situation like this has happened, the server has been operational since 2014. It is a private, low volume e-mail server as well, not something with external public users. The reason for the delay in recognizing an issue existed is that same domain email was working as were the other internet facing services. The firewall had choked on state tracking of port 25 but was otherwise functional.