One of my clients that I'm constantly doing business with - starting a week ago - begin not receiving my E-mails in time - When I say - not in time - means that they will receive my E-mails about 24 h later.
I didn't change anytihing in our mail server in the past week.
Eventualy after couple of hours I am getting a response back from them saying:
"421 invalid sender domain, possibly misconfigured"
I did run several searches on the Internet related to this issue and somewhere there was an explanation like this error denotes a temporary delivery failure. "This error indicates to your mail server, that it should attempt to resend the email later"
I did email to the e-mail address in question from two different places and it works great and fast.
From my domain - does not work. I am not blacklisted in any server.