spost(8) has been merged into post(8). Its functionality is

enabled by selecting the "pipe" mail transport method in
etc/mts.conf, which in turn can be configured using
--with-mts=pipe.  The spost -noalias, -backup/-nobackup,
-push/-nopush, and -remove/-noremove switches are not
supported by post.  Note that spost did not support -whom or
Dcc, and neither does post when using mts: pipe.  spost is
now deprecated.