Determine ISP port blocking?

I have setup my at home email server for external access on an odd, high
port number. It has been working well with dynamic DNS from my domain
provider and everything. Then, it suddenly stopped answering and I can
get into it, even with the specific IP.

A brief check of all configuration and operation this morning showed
that all is supposedly well on my end. Is there a way I can tell if my
ISP is suddenly blocking the ports?

Alan