When sendmail finds multiple A records for a host (and no MX records), it tries them in the order returned by DNS. DNS usually returns the A record that is on the same network first. The sendmail program assumes that DNS returns addresses in a useful order. If the address that sendmail always tries first is not the most appropriate, look for problems with DNS, not with sendmail.
There is no way to discover that another site has used yours as a disaster MX site unless someone at that other site tells you. Instead, you may one day suddenly discover many queued messages from outside your site destined for some host you've never heard of before.
Under old versions of DNS an error in the zone file causes the rest of the file to be ignored. The effect is as though many of your hosts suddenly disappeared. This problem has been fixed in 4.8.3 and 4.9.x.
Sites with a central mail hub should give that hub the role of a primary DNS
server. If /etc/resolv.conf contains
localhost as its
first record, lookups will be faster. Failure to make the mail hub
a DNS server runs the risk of mail failing and queueing when the
hub is up but the other DNS servers are down or unreachable.
Prior to V8.8 sendmail the maximum number of MX records that could be listed for a single host was 20. Some sites, such as aol.com, will reach that limit soon and exceed it. Beginning with V8.8 sendmail, that maximum has been increased to 100.