A coalition of AOL, BT, Comcast, Earthline, Microsoft and Yahoo calling itself the “Anti-Spam Technical Alliance” has published a best practice document addressed to ISPs and mailbox providers.
This “technical and policy proposal” contains many well-established matters of best practice (like closing open relays), some newer ones (like proactively monitoring for and disconnecting compromised PCs connecting to the network), and some which might be considered controversial (like requiring outgoing mail to be subject to hourly and daily rate limits based on the number of recipients in To/Cc/Bcc fields).
There is also some heavy promotion of the use of mail submission port 587 and SMTP AUTH in preference to the same with SMTP port 25 when done by users relaying through off-network mail servers. This per RFC 2476 and RFC 2554. Apart from customers specifically identified as needing to run their own mail server, the coalition say, customer use of port 25 should be blocked.