pts/1:makoto@serv 20:55:36/210314(~)> echo test | mail -s test-from-serv root
pts/1:makoto@serv 20:56:18/210314(~)> WARNING: RunAsUser for MSP ignored, check group ids (egid=1000, want=17)
can not chdir(/var/spool/clientmqueue/): Permission denied
Program mode requires special privileges, e.g., root or TrustedUser.

pts/1:makoto@serv 20:56:22/210314(~)> id smmsp
uid=17(smmsp) gid=17(smmsp) groups=17(smmsp)
pts/1:makoto@serv 20:56:48/210314(~)>  ls -nla /usr/pkg/libexec/sendmail/sendmail
-r-xr-sr-x  1 0  1000  603690 Jan 10  2010 /usr/pkg/libexec/sendmail/sendmail*
pts/1:makoto@serv 20:59:06/210314(~)> 

pkg install may look at NIS database (as natural), but when running sendmail, it will not honor NIS database instead of looking local vipw info, (I'm afraid)

pts/1:makoto@serv 20:59:06/210314(~)> sudo chown 17:17 /usr/pkg/libexec/sendmail/sendmail
Password:
pts/1:makoto@serv 21:04:10/210314(~)> sudo chmod 2555 /usr/pkg/libexec/sendmail/sendmail
pts/1:makoto@serv 21:04:31/210314(~)> ls -nla /usr/pkg/libexec/sendmail/sendmail
-r-xr-sr-x  1 17  17  603690 Jan 10  2010 /usr/pkg/libexec/sendmail/sendmail*
pts/1:makoto@serv 21:04:34/210314(~)> 

Now, no error is shown:

pts/1:makoto@serv 21:04:34/210314(~)> echo test | mail -s test-from-serv root
pts/1:makoto@serv 21:05:34/210314(~)>