The SMTP HELO command is used to
introduce the calling machine to the receiving machine. The form of
that command is:
HELO calling host name here
Note that HELO and EHLO are equivalent in this regard. Ordinarily,
sendmail doesn't care what the
calling host calls itself. All sendmail cares
about is that this name is the canonical name of a machine. If you
care whether the HELO hostname matches the real hostname of the
calling machine, you can add a line such as the following to your
Build m4 file:
APPENDDEF(`confENVDEF', `-DPICKY_HELO_CHECK')
With PICKY_HELO_CHECK defined, a mismatch (other than the local
machine calling itself localhost) will cause the
following warning to be logged:
Host realname claimed to be heloname
Note that this check is ordinarily turned off because a large number
of hosts on the Internet use a name that is different from their
canonical name.
