sendmail, 4th Edition

When a mail message arrives that includes an ORCPT parameter for the ESMTP RCPT command (see RFC1891), sendmail needs to save that parameter's information separately from the RCPT recipient address:

RCPT TO:<gw@wash.dc.gov> ORCPT=rfc822;gw@wash.dc.gov recipient address parameter's information

Not all sites understand DSN. If sendmail forwards the message to such a site, it needs to omit the ORCPT parameter. Consequently, sendmail must not store that parameter with the RCPT address.

The Q line is used to separately store the ORCPT parameter information:

Q type ; addr

The type ; addr is defined by RFC1891. The sendmail program checks the syntax of addr when that information is received but otherwise merely stores type ; addr as is in the Q line.

There must be only a single Q line for each recipient R line, and each such Q line must precede its corresponding R line.