When another host connects to the local host and that other host claims to have the same canonical name as the local host, it should be considered an error. In V8.6 sendmail, setting the CheckLoopBack variable in conf.c determined whether this error was detected. But beginning with V8.7 sendmail, this check is based on the delivery agent. If the F=k delivery agent flag is absent, the check is done. If the F=k delivery agent flag is present, the check is skipped.

The check is performed only for SMTP connections. The literal canonical name given in the connecting host's HELO or EHLO response is compared to the canonical name for the local host. If they are the same, the following error is printed, and the connection is disallowed:

553 5.3.5 host config error: mail loops back to myself V8.6 
553 5.3.5 host config error: mail loops back to me (MX problem?) V8.7 and above 

Here, host is the name of the offending host.

A problem can arise at sites that run two different invocations of sendmail (one for SMTP connections and another for command-line invocation, where each uses a different configuration file). In this instance, when the latter connects to the former, this error can occur. Such sites might find it necessary to set the F=k delivery agent flag for the delivery agent that handles SMTP connections (usually smtp). With an mc configuration, the following command does just that:

define(`SMTP_MAILER_FLAGS',`k')             prior to V8.10
MODIFY_MAILER_FLAGS(`SMTP', `+k')           V8.10 and above
MAILER(`smtp')                              must be last

Note that sendmail must recognize its local hostname among many possible names. See $=w for a discussion of $=w and MX records.