some incoming emails failing?

[its-surrey]

mydomain is registered with BT, MX record points directly to my exchange server.  Has been working fine for months and months but today we've had 3 or 4 different companies emails fail with the failure below.  Any ideas?

Reporting-MTA: dns; c2bthimr01.btconnect.com
Arrival-Date: Mon, 7 Jan 2013 15:01:39 GMT

Final-Recipient: RFC822; user@mydomain.co.uk
Action: failed
Status: 5.1.1
Remote-MTA: DNS; mydomain.co.uk
Diagnostic-Code: SMTP; 550 relay not permitted
Last-Attempt-Date: Mon, 7 Jan 2013 15:01:44 GMT

[adrianc]

Hi,

I would double check that the MX record is still pointed to your server. It is possible that there's been an error of some sort on the back-end systems and it's changed.

If the MX is still pointed to your server then check the logs to see if there's anything interesting in there.

Adrian