Frequent reports of NDRs with 5.7.1 errors

Noticed an increase in NDRs with 5.7.1 error from posts in newsgroups and a colleague saw 2 of those within the last week. If you're using Exchange, you may see Event IDs 1709 and 1710 in your Application Log. Some common causes:
1) Allow computers which successfully authenticate to relay checkbox not checked on SMTP virtual server
2) DNS misconfiguration and Recipient Policy issues
3) Proxy addresses created manually that do not match any Recipient Policy (I was surprised by this one!)
4) If using ISA server, ISA's IP address changes but server publishing rule not changed

More info about what causes these on the Exchange site.