We are a small ISP and have suddenly in the last few weeks had a problem where customers cannot send email to PlusNet domains from our SMTP relay servers.
The relay servers are clean - not on any RBLs - PTR records in place, correct HELO etc.
Using Cisco's senderbase.org shows shows the servers reputation as 'Good'.
No problems sending to anybody else except domains hosted by Plusnet.
Phoned PlusNet to discuss the problem, hoping for a pointer and met with total disinterest - 'ask our customer to contact us'.
Has anybody else seen this - if so any clues. All has been fine until about two weeks ago. None of our relays can send mail to plus.net.