The User Database (See this section) is code inside sendmail that allows sender and recipient addresses to be rewritten under the control of an external database. This code is automatically included in sendmail when you define NEWDB or HESIOD:

APPENDDEF(`confMAPDEF', `-DNEWDB')   automatically include User Database code
APPENDDEF(`confMAPDEF', `-DHESIOD')  automatically include User Database code

If you don't want to include support for the User Database, you need to specifically turn it off by setting USERDB to 0:

APPENDDEF(`confMAPDEF', `-DUSERDB=0')

See the UDB_DEFAULT_SPEC compile-time macro (UDB_DEFAULT_SPEC) for a method to set a default for the database location.

If you are running a precompiled sendmail binary, you can use the -d0.1 debugging command-line switch (-d0.1) to determine if USERDB support is included (if it appears in the list, support is included).