When using Forefront Protection for Exchange, a legitimate sender gets the following bounce back when trying to e-mail you:
“550 5.7.1 :127.0.0.3:Client host … blocked using 88.blocklist.zap; Mail from IP banned. To request removal from this list please forward this message to firstname.lastname@example.org”
If you are sure that the sender is legitimate and trusted, mail from the senders domain can be allowed by completing the followings solution.
Add the IP address of the remote domain to the “IP Allow List” on your Edge servers. (This IP can be found in the NDR or by contacting the remote party):