Not much you can do on your end. Your ISP is the one blocking other mail servers. It's kind of like getting a call where someone says they are from company X, but you want to be sure they aren't lying to you. So you check the caller ID and then do a reverse lookup to see who that number belongs to. If it's an unlisted number you cannot verify they are legit, so you give them an error message and hang up. In this case your ISP is doing that on your behalf before sending anything on to you.

It's really the responsibility of anyone running a mail server to make sure it complies with all the latest RFCs. There is a 100% chance that these people who cannot email you now, also have issues with other people using other ISPs.