My guess is that the domain has a valid MX record but for some reason the server isn't currently accepting messages.
Another possibility is that the server is having technical difficulties. I recently had problems with BT internet. It was because their servers often open two or three SMTP connections just to send one message and for some reason my server was configured to only allow a single connection per server.
I've no idea why BT's email provider is doing that. They don't do anything with the extra connection(s). And it doesn't really make sense that they drop all the connections just because my server refused some of them.
Edited by Andrue (Thu 06-Jul-17 17:35:56)