Old BSD-based versions of Unix, such as SunOS 4.0.x and BSD 4.3, used the signal(2) and sigsetmask(2) calls to set and release signals. Modern versions of Unix use the sigaction(2) and sigprocmask(2) pair of routines. For all currently supported systems, BSD_3 is already correctly defined in the devtools/OS files or in sendmail/conf.h. You should need to define BSD_3 if you are porting to a previously unsupported, old BSD-based system:

APPENDDEF(`confENVDEF', `-DBSD4_3')

When porting to a new system, you can test with the preceding confENVDEF statement and, if successful, put a permanent porting entry into sendmail/conf.h. New ports should be reported to sendmail@sendmail.org so that they can be folded into future releases.