The "smtpd" command:

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.