Ordinarily, sendmail will first look for a service-switch file (ServiceSwitchFile) to see how it should look up its aliases. If it finds one, and if the service term aliases is listed in that file, it uses the techniques listed following that term to look up its aliases. In the absence of a service-switch, or if the service-switch could not be opened, sendmail's fallback position is to use the technique called files to look up its aliases.

This AUTO_NIS_ALIASES definition, when specified during compilation, also causes sendmail to automatically add the technique nis if NIS was defined or nis+ if NISPLUS was defined:

APPENDDEF(`confMAPDEF', `-DNIS -DAUTO_NIS_ALIASES')
APPENDDEF(`confMAPDEF', `-DNISPLUS -DAUTO_NIS_ALIASES')

The first line causes the fallback list of techniques to become files then nis, and the second causes it to become files then nisplus. Note that AUTO_NIS_ALIASES is not defined in any devtools/OS files distributed with sendmail.

If you are running a precompiled sendmail binary, you can use the -d0.10 debugging command-line switch (-d0.10) to determine if AUTO_NIS_ALIASES support is defined (if it appears in the list, it is defined).