I run a relay, or I did until yesterday.
I suppose you know about this problem, new for me. Mail from accounts on my server addressed to hotmail.com and live.com addresses suddenly stopped being delivered. The bounce message from Microsoft claimed I was on a block list.
I checked, and the only list I am on is the one run by https://www.dan.me.uk/dnsbl called *.tor.dan.me.uk *which reports every ip address participating in Tor, including relays. He warns against using the list to block mail, but Microsoft seems to be doing it anyway.
As soon as I stopped the relay, mail began to be accepted again within an hour or so.
I have had to shut off my relay because people are complaining they can't send mail to their friends/relatives on hotmail or live addresses.
This is a difficult dilemma for me. Much as I want to continue to run the relay, the people using my mail server aren't going to be agreeable to being told they can send mail to friends on hotmail.
I wrote also to the guy, Dan, in the U.K. to tell him his list is being used in this way.