The MAIL FROM: command normally uses the envelope address for the sender:

MAIL From:<jqp@wash.dc.gov>

If the F=p delivery agent flag is specified, sendmail instead sends a transformed version of that address. The transformation can take one of two forms, depending on the first character of the envelope address. If that address begins with an @ character, an @, the local hostname, and a comma are prefixed to that address to create a legal return path:

<@hub:jqp@wash.dc.gov>
  
 
becomes
  
 
<@ourhost,@hub:jqp@wash.dc.gov>

If the envelope address for the sender does not start with an @ character, an @, the local hostname, and a colon are prefixed to that address:

<jqp@wash.dc.gov> becomes <@ourhost:jqp@wash.dc.gov

See also the DontPruneRoutes option (DontPruneRoutes).

Note that these forms of address transformations are discouraged by RFC1123. For this reason the F=p delivery agent flag is deprecated and might be removed from future versions of sendmail.