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.