RFC974 says that when mail is being sent from a host that is an MX record for the receiving host, all MX records of a preference equal to or greater than the sending host must be discarded. In some circumstances this can leave no usable MX records, and if that is the case, V8 sendmail bases its action on the setting of its TryNullMXList option.

The forms of the TryNullMXList option are as follows:

O TryNullMXList=bool               configuration file (V8.7 and later) 
-OTryNullMXList=bool               command line (V8.7 and later) 
define(`confTRY_NULL_MX_LIST',bool)    mc configuration (V8.7 and later) 
Owbool                             configuration file (deprecated) 
-owbool                            command line (deprecated) 

The bool is of type Boolean. If it is false, sendmail bounces the mail message with the following error message:

MX list for otherhost points back to thishost 

If bool is true, sendmail looks to see whether the receiving host has an A or AAAA record. If it does, sendmail tries to deliver the mail message directly to that host's A or AAAA record address. If the host doesn't have an A or AAAA record, sendmail bounces the message. See Section 9.3.7 in Chapter 9 for a full discussion of why one setting might be preferable over another. Note that RFC2821 requires that this option be set to false to prevent it from creating unpredictable mail routing. The default with the mc configuration technique is false.

The TryNullMXList option is not safe as of V8.8.4. If it is specified from the command line, it can cause sendmail to relinquish its special privileges.