The "smtpd" command:

Usage: fossil smtpd [OPTIONS] REPOSITORY

Begin a SMTP conversation with a client using stdin/stdout.  The
received email is stored in REPOSITORY.

Options:

     --dryrun          Do not record any emails in the database

     --trace           Print a transcript of the conversation on stderr
                       for debugging and analysis

     --ipaddr ADDR     The SMTP connection originates at ADDR.  Or if ADDR
                       is the name of an environment variable, the address
                       is taken from that environment variable.