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
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.