I just tested with four separate West coast ISPs and none of them are blocking outgoing port 25. Maybe it’s a regional thing

They can’t really stop you from sending emails out without potentially breaking legitimate mail. They can stop email from being submitted to you by blocking port 25/465/587 inbound though.

I did something similar with Graylog and ssh login attempts. Even had a neat little real-time global map with dots on the location the attempts originated from.

I get surprisingly few login attempts on my mail server though.