We've just updated our network settings and everything is working ok apart from now we can't send or receive emails.
RDP is working and telnet on port 25 is getting a response from the server.
The network change was the IPs on the external network connection of the server as we've moved ISP. We updated the forwarders in DNS and have also updated the SMTP relay allow list within Exchange.
When trying to send email we receive an Undeliverable message from the exchange server saying Recipient address rejected: Relay access denied. Test emails sent in never arrive.
Incoming email is passed through an external SPAM service and the delivery IP was updated on the control panel and the test from there says that it's working ok.
Any help greatly appreciated as we can't seem to track down the issue but believe it either to be something missed in Exchange or something we've overlooked in ISA server.