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Confusing SMTP mechanism

We just started using SMTP relaying through our mail server to replace Personio mailbox by an internal one when we respond to an incoming mail.
Our setting is the public IP address of our exchange server with port 25 and non-encrypted mail transfer.
When we introduced this setting we needed to put a policy in place on our firewall. First aspect of this policy was to accept only connections from the 3 known Personio mail servers. Due to tests with encryption we started with a couple of open ports like 25, 465, 587, ... This worked fine especially for the setting I wrote above and so we decided to leave it for the moment. But now we wanted to restrict our firewall following our security guidelines to the needed rules and we cut the list of open ports to the only left port 25. When we did this, for our surprise the smtp connection timed out from this moment.
So we rolled back and had a look into our log and figured out the currently needed setting... For some reason it seems to be part of the Personio communication process, that port 465 needs to be open at the start of each SMTP communcication. It will not be actively used, but when we have it open, the communication starts on port 25 as configured, when we close it, the communication in Personio times out and there is nothing more to be seen in our Deny-Logs our somewhere else.

So my questions are? What is this port used for? Why doesn't Personio used the port we configured in our settings? Is this documented somewhere or do I do I have to hope that this behaviour never changes to another "mysterious" port like 587 or other? Could this be an overcome of our tests with SMTPS that still remains in our companys profile settings?

Thanks and best regards