diff --git a/mail/sendmail/sendmail.conf b/mail/sendmail/sendmail.conf
index e85ede7d4..b88603814 100644
--- a/mail/sendmail/sendmail.conf
+++ b/mail/sendmail/sendmail.conf
@@ -36,6 +36,9 @@ sm_main() {
 	#
 	cp $confdir/site.config.m4 ../devtools/Site/
 
+	# IPv6 support by default
+	sm_appenddef "confENVDEF" "-DNETINET6=1"
+
 	# SASL2 (smtp authentication)
 	if pkginstalled -f cyrus-sasl2; then
 		sm_appenddef "confENVDEF" "-DSASL=2"