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:

Qtype;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.