Make sendmail picky about HELO tune with confENVDEF

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:


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.[7]

[7] Eric was getting complaints that the continual insertion of this warning was misleading and tended to cause people to ignore it entirely.

    Part I: Build and Install
    Chapter 2. Build and Install sendmail
    Chapter 4. Configure sendmail.cf with m4
    Part II: Administration
    Part III: The Configuration File