A problem developed on the email server today that caused it to bounce some email.
The problem was that the internal email delivery agent for email destined to our server stopped doing DNS lookups properly and so could not resolve destination domains. I’m not sure exactly what caused this to happen however I have found the necessary configuration options for postfix to force the name resolution to do what it needs to and things are now fixed.
The fix for this is to set:
smtp_host_lookup = dns, native
lmtp_host_lookup = dns, native
Unfortunately the email that was bounced as a result of this will have been “returned to sender”.