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. Nevertheless, some broken user agents and mailers (e.g., many older versions of sendmail) will emit such illegal messages.
missingrecipientpolicy channel keyword takes an
integer value specifying what approach to use for such messages; the
default value, if the keyword is not explicitly present, is 0, meaning
that envelope To: addresses are placed in a To: header.
|0||Place envelope To: recipients in a To: header|
|1||Pass the illegal message through unchanged|
|2||Place envelope To: recipients in a To: header|
|3||Place all envelope To: recipients in a single Bcc: header|
|4||Generate a group construct ( i.e., ;) To: header, To: Recipients not specified|
|5||Generate a blank Bcc: header|
|6||Reject the message|
Note that the MISSING_RECIPIENT_POLICY PMDF option, discussed in Section 7.3.1, can be used to set a PMDF system default for this sort of behavior.