I am hosting my own email server. It can receive mail on port 25, but unfortunately my ISP blocks sending on that port. so I chose your "SMTP Outbound Relay" service, which I use on port 26. it works, but then I can no longer send mail. How can I resolve this issue? Should I change my service from Dynu or is it a configuration issue?

If its like my ISP, they block port 25 is both directions, except to their SMTP server.
On my mail server I have a "Smart Host / Outbound Mail Gateway" configured that point to the ISP's mail server and it relays all my email. It does require a login, but is transparent to your domain email addresses.

Basically you have to SMTP relay in (Dynu) and SMTP relay out (Your ISP SMTP server).

I do not know how to receive any outside inclination.
what should I do ?