Prior to V8.7 sendmail, only the local delivery agent could recognize the :include: directive for creating mailing lists (Section 13.2).

Beginning with V8.7, any delivery agent can be made to recognize the :include: directive by setting the F=: delivery agent flag (or to ignore it by not setting the F=: delivery agent flag). This delivery agent flag allows you to design a local delivery agent without :include: support or local-type clones with :include: support. In configuration files prior to Version 6 (Section 17.5), this delivery agent flag is automatically set for the local delivery agent.

This delivery agent flag is legal only for addresses contained in ~/.forward files or aliases(5) databases. Any :include: address in an SMTP dialog or on the command line will be rejected.