RFC2821 requires that all sites be set up so that mail addressed to the special name Postmaster^[47] always be successfully delivered. This requirement ensures that notification of mail problems can always be sent and successfully delivered to the offending site.^[48] At most sites the name Postmaster is an alias to a real person's name in the aliases file. Mail to Postmaster should never be ignored.

^[47] The name Postmaster is case-insensitive. That is, POSTMASTER, Postmaster, postmaster, and even PoStMaStEr are all equivalent.

^[48] Note that adoption of RFC1648, entitled Postmaster Convention for X.400 Operations, has extended this concept to include hosts addressed as user@host.domain that are really X.400 sites masquerading as Internet sites.

Ordinarily, notification of locally bounced mail and other mail problems is sent back (bounced) to the sender of the message. The local person in the role of Postmaster does not get a copy of local failed mail.

The PostmasterCopy option tells sendmail to send a copy of all failed mail to another person, often Postmaster. Under V8 and SunOS that copy contains only the failed message's header. Under very old versions of sendmail, that copy includes both the header and the body.

The forms of the PostmasterCopy option are as follows:

O PostmasterCopy=user            configuration file (V8.7 and later) 
-OPostmasterCopy=user            command line (V8.7 and later) 
define(`confCOPY_ERRORS_TO',user)    mc configuration (V8.7 and later) 
OPuser                           configuration file (deprecated) 
-oPuser                          command line (deprecated) 

The argument user is of type string. If the argument is missing or if the PostmasterCopy option is entirely missing, no extra copy is sent. The default for the mc configuration technique to not send an extra copy.

While debugging a new sendmail.cf file, it is wise to define the PostmasterCopy option so that you receive a copy of all failed mail. Once the configuration file is stable, either the PostmasterCopy option can be removed or the name can be replaced with an alias to a program. Such a program could filter the copies of error mail so that only serious problems would be seen.

Macros used in the user argument will be correctly expanded before use. For example:

D{NOTIFYHOST}mailhost                  beginning with V8.7
O PostmasterCopy=Postmaster@${NOTIFYHOST}  beginning with V8.7

DAmailhost                             deprecated
OPPostmaster@$A                        deprecated

The PostmasterCopy option is not safe. If specified from the command line, it can cause sendmail to relinquish its special privileges.