Prior to V8.12, if you were running sendmail as a daemon, you needed to define SMTP to enable mail transfers. If you didn't intend to run sendmail as a daemon, SMTP did not need to be defined. The default was that SMTP was automatically defined if either NETINET or NETISO was defined; otherwise, SMTP was undefined.

Beginning with V8.12, the SMTP compile-time macro has been deprecated and removed. It is now impossible to exclude SMTP support from sendmail.

If a precompiled sendmail lacks SMTP support, an attempt to use sendmail's -bs command-line switch will result in this fatal error:

I don't speak SMTP

SMTP activity can be watched with the -v command-line switch (-v).