I've just edited and anonymised my last post in case I can't control myself here...
If you haven't read it, you should read that one before this one.
We've had no SMTP inbound mail since lunchtime yesterday.
We contacted our ISP (PT) but they weren't interested in the problem. They told me to reboot the Domino server....
Internal mail is working fine.
I tried using a VB Application to send SMTP mail to the server's internal IP Address. It worked fine - proving that if mail were reaching the server it would process it.
All indications were that there was a duplicate IP on the internet - but that couldn't be right - we got that problem fixed yesterday...
After hours of wasted IT time, we finally discovered the truth. Our ISP (PT) who duplicated our IP Address yesterday had removed the address as requested. They then put the address back into their system and allocated it to a new customer.
I can't think of a closing statement that wont get me in trouble with the law so I'll leave it up to your imaginings.