Most of the time my clients have two priorities.
1: Get rid of all spam
2: Never loose an email that is important to business
The unfortunate thing is that these two priorities are more often then not in conflict with each other.
The same Spam filter that will block every spam message will also catch a few messages that *should* go through. I have not worked with a spam solution that automatically knows what is passable and what is to be blocked.
So this is the conundrum that the administrators of the email system need to work within. How tight do you keep the spam filters and how many valid email messages are you willing to lose or do you loosen the spam filters and have an ‘acceptable’ quantity of unsolicited commercial email?
This tend to be different for every person at every client site, but I do know that no matter what decision are made today, tomorrow a user will be upset because of too much spam or missing an email message that was important. Sometimes it is even the same person on the same day …..