Whether to enable the submission service configured in master.cf.
This service listens for plain text SMTP connections on port 587
and supports explicit TLS via StartTLS.
It is a way for clients to login and submit mails after an inband
connection upgrade using StartTLS.
::: {.warning}
[RFC 8314](https://www.rfc-editor.org/rfc/rfc8314) discourages the use
of explicit TLS for mail submissionn.
:::