So I’m a little unclear how you send the mail to your host on port 25. Why I say that is I have a postfix server up and running within an Arch Linux VM. The VM has a fully resolvable host name or URL with SSL certs running on either ports 465 or 587. Since it’s internal I have a DNS override at the router level to redirect requests for the URL to the machine running postfix.
I’m not sure how you are trying to access postfix. I think your problem is going to come down to your SMTP client and server settings to best answer this question.