The $r sendmail macro holds as its value the protocol that is used in receiving a mail message (usually SMTP or UUCP, $r). The $s sendmail macro holds as its value the name of the sending host ($s). Some programs, such as UUCP, need to be able to set the values of these macros from the command line. The old way to set them looked like this:

-oMrUUCP -oMslady

Here, the M option sets $r to be UUCP and $s to be lady.

Under V8 sendmail the setting of $r and $s has been simplified. A command-line single switch, -p, can be used to set them both:

-prval:sval  

Here, the rval is the value assigned to $r, and the sval is the value assigned to $s. The two are separated by a colon. If the sval is omitted, the colon should also be omitted.