The Return-Receipt-To: header should never be declared in the configuration file and, in fact, should rarely be used at all. It is not intended as a routine delivery-verification mechanism, but rather is intended for occasional use in debugging delivery problems. It is especially dangerous when used in outgoing mailing-list mail because it can cause an avalanche of returned mail and can possibly bring a host to its knees.

Beginning with V8.6 sendmail, a receipt is sent when the mailing list is first expanded, and the Return-Receipt-To: header is removed before forwarding the message to the list.

Beginning with V8.7 sendmail, processing of all Return-Receipt-To: headers can be skipped by specifying noreceipts with the PrivacyOptions option (PrivacyOptions).

Return notification is triggered by NOTIFY=SUCCESS extension (-N) to the RCPT TO: command. If the PrivacyOptions option's noreceipts (See this section) keyword is specified, no success return notification will be sent. Beginning with V8.10, if the RrtImpliesDsn option (RrtImpliesDsn) is set, the presence of a Return-Receipt-to: header will cause sendmail to act as if NOTIFY=SUCCESS was specified, even if it was not. In this instance, the value of the Return-Receipt-to: header is ignored. Other than with the RrtImpliesDsn option, the Return-Receipt-to: header is otherwise ignored.