As a
part of the STARTTLS form of authentication and encryption,
certificates are usually exchanged. If a certificate is presented by
the other side, this ${cn_issuer} macro is
assigned the CN of that certificate. That value might look like this:
Juan+20Garcia 
a person's name
Garcia's+20Software a business name
See Section 21.6.2 to find how and why the value in
this macro undergoes special translation.
${cn_subject} is transient. If it is defined in
the configuration file or in the command line, that definition can be
ignored by sendmail. Also note that a
$& prefix is necessary when you reference this
macro in rules (that is, use $&{cn_subject},
not ${cn_subject}).
