I have a user who is using Hotmail to communicate with one of our suppliers.
Recently, Hotmail started bouncing emails to that address (while other ISPs are working fine.) Hotmail bounce messages contain very little details.
Investigating, I found that the supplier's ISP has appeared on a blacklist of ISPs who do not do enough to prevent spamming from their users. (Note: Neither my user nor the supplier are mass-mailing.)
In the circumstances, I can understand Hotmail declining to accept emails from the host, or accepting them but being quick to mark them as spam. However, I am surprised that they might refuse delivery.
Does this sound plausible to you, or am I barking up the wrong tree?