Spam Filtering on Forwarded Mail

The problem

Our mail server became clogged with messages that it is trying to deliver to yahoo.com e-mail addresses. The mail server at yahoo.com deferred them for days, refusing to accept them at all, or only accepting them after a delay of a couple of days.

Why did yahoo.com defer the messages? Although yahoo.com keeps its security practices hidden, it appears that it began to see our mail server as a spam relay. The issues appear to be:

The issues

The bottom line is that there's no point in forwarding or relaying spam through our mail server. The only way I know to prevent this is to scan e-mail for spam as we receive it, which means implementing some sort of spam-rejection procedure for everyone at Nevis, not just for those who forward their mail.

The tool we use to scan messages for spam is SpamAssassin. Up until now, although every e-mail received at Nevis has been scanned by SpamAssassin, I've left it up to each user to decide whether they wanted to use it. Now I've taken away some of that choice, as described below.

The policy I am putting into place is the same policy as that used by Columbia University to filter their e-mail. In fact, I am being more generous than Columbia. However, I realize the issues involved:

The solutions

Here are the new policies/solutions: