The $r sendmail macro ($r) holds as its value the protocol that was used when a mail message was first received. That value is either SMTP, ESMTP, or internal, or it is a protocol assigned with the -p command-line switch (-p). If $r lacks a value, this proto= syslog equate is omitted. If $r has a value, the first 20 characters of that value are printed following the proto= in the syslog line:

proto=ESMTP