Funny, OpenBSD provides a greylister, spamd. However, the article does not mention it.
Spamd can tar pit hosts on the blacklist. Blacklisted hosts are sent to a separate dummy SMTP server that responds slowly and discards everything.
http://www.openbsd.org/spamd/ has details.
Thank you for including configuring submission protocol.
You neglected the "Why"
Many ISPs block outbound SMTP, but permit outbound submission protocol.
May I suggest a followup article.
Mail senders that do not play nice with greylisting.
There is a well loved advertising company that retries sends from EVERY host in their netblock.
There are several parcel carriers that attempt delivery only once.