Defining MATCHGECOS causes code to be included inside sendmail for support of limited fuzzy name matching. This process is described under the MatchGECOS option (MatchGECOS). This MATCHGECOS compile-time macro is normally defined as true by default. If you want to turn it off, use an expression such as this in your Build m4 file:

APPENDDEF(`conf_sendmail_ENVDEF', `-DMATCHGECOS=0 ')
                                                 
                              disable fuzzy name matching inside sendmail

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