The F=C delivery agent flag causes sendmail to append an @domain extension to any recipient address that lacks one after having been rewritten by the canonify rule set 3. The @domain that is added is copied from the envelope sender's address.

This F=C delivery agent flag is not looked for in the delivery agent definition that was selected to send the message. Rather, it is looked for in the delivery agent that would be selected if the sender were the recipient (as in the case of bounced mail).

To illustrate, consider the following mail:

From: bill@oursite.edu
To: john@remotesite.gov, alice

The recipient address alice lacks an @domain specification. The sendmail program processes the envelope sender address bill@oursite.edu to decide on a delivery agent definition that can be used if this mail needs to be returned. If that envelope sender's return mail delivery agent has the F=C delivery agent flag set, the @oursite.edu part of the envelope sender's address is appended to alice:

From: bill@oursite.edu
To: john@remotesite.gov, alice@oursite.edu

The F=C delivery agent flag is traditionally used for the smtp class of delivery agent that is supposed to always supply an @domain part for all addresses.

Note that the domain part of the envelope sender is used. In our example the envelope and header sender are the same.

See also the always_add_domain feature (FEATURE(always_add_domain)).