We are running SME 9.2, with updates and contribs. There is one client whose email is not arriving to our server. I cannot find any evidence of a rejection in the logs. They are receiving email from us, and we can receive email from them using a gmail account. We are sending and receiving lots of email to and from others.
On the sender's end, they are getting a bounce message saying the email could not be delivered. Not other information is provided. No rejection header, for example. The sender uses a Windows based email server but I don't know the name of it.
Finally, this began recently, say in the past seven days. We have receiving email from them for years.
What might cause a single domain to be blocked?