What about the other ports? I would be particularly curious about BCP
for ports 465 (ssmtp, SSL-wrapped SMTP) and 587 (submission, often
used for plain-text SMTP with an immediate STARTTLS required).
Can a case be made for spam reduction by blocking the ports for
outgoing connections? Are there many systems around that accept
incoming mail on these ports?
I believe that this is something that should definitely be
addressed in a BCP. However, I do not know if this is something that
is already covered by an existing BCP -- I'll have to do some
searching.