Missing recipient policy MTA option

Alias and address MTA options:  (0-6)
RFC 822(Internet) messages are required to contain a recipient header: a To:, Cc:, or Bcc: header. A message without any such header is illegal according to RFC 822. Nevertheless, some broken user agents and mailers (e.g., many older versions of sendmail) will emit such  illegal (per RFC 822) messages. Note that RFC 5322, the update to RFC 822, relaxes the RFC 822 requirement and allows submitted messages to be lacking in any recipient header line. However, unless it is certain that all the MTAs and MUAs that may ever handle a message in  fact conform to RFC 5322 (rather than the older RFC 822), it is unwise  to emit a message lacking all recipient header lines, since the  behavior of an RFC 822-compliant MTA or mail user agent may be  undesirable when encountering a message that is, from its point of  view, illegal---results may include rejection of such a message,  potentially undesired exposure of recipient information such as  recipients intended as Bcc: recipients, etc.

The   MTA option takes an integer value specifying what  approach to use for such messages; the default value, if the option is  not explicitly present, is 0. The meaning of this default value of 0 has changed: prior to MS 6.2, it was equivalent to 2, meaning that  envelope To addresses are placed in a To: header line. As of MS 6.2, it is equivalent to 1, meaning to pass such messages through unchanged, in  accordance with what RFC 5322 now recommends.

Note that the    channel option can be used to set per-channel controls  for this sort of behavior; such per-channel controls override the  setting of the MTA option.

See also:
 * missing_recipient_group_text MTA Option
 * missingrecipientpolicy Option
 * acceptalladdresses Option
 * Alias and address MTA options