When V8.7 or above sendmail has been compiled with User Database support (userdb), you can specify this delivery agent flag for a delivery agent and thereby cause that delivery agent to perform a User Database lookup for each address it handles. For sender header and envelope addresses, a User Database mailname keyword is used to perform reverse aliasing. For recipient envelope addresses, a User Database maildrop keyword is used to perform additional forward aliasing.^[25] Note that any address with a leading @ character (in the $: part of the triple returned by the parse rule set 0) causes User Database lookups to be skipped. Also note that the absence of an F=i delivery agent flag (F=i) suppresses User Database rewriting of the envelope sender.

^[25] See F=j (F=j) for User Database lookups of header recipient addresses.

If the F=@ delivery agent flag is present, the delivery agent will try to use the User Database. If the F=@ delivery agent flag is absent, all User Database lookups are skipped. In general, this delivery agent flag should be present for the local and local-clone delivery agents but absent for all others. In configuration files prior to Version 6 (Section 17.5), this delivery agent flag is automatically set for the local delivery agent.