I use spam assasin on my front end which catches 98-99% of my companies spam. The remaining 1-2% of mail gets treated to SpamLion (http://www.spamlion.com). SpamLion uses smart sender validation. A first time sender will recieve a challange mail to validate that they are a real person and not a spammer. Once they respond their email will go through to the users inbox. After that, they never have to validate again. Obviously a user can decide to block them. Users can also go through their personal quarantine and release items.

Some people do not like sender validation because they think it floods the internet with validation mails. If you set it up right and put spam assassin in front of it there is no problem. I have it setup so that an email address will only recieve a challenge mail once every 30 days so if a spammer is using it to send out millions of emails the mail address will never get bombarded. Also whenever a user sends out an email it gets automatically white-listed so a challenge mail is never sent.

After about 30 days most users stop checking the SpamLion quarantine as by this time their normal contacts have all been white-listed.

As a mail admin, I must say it is nice not to have to update my rules with the latest spelling of Viagra every week to keep spam out.

It is true that nothing beats the human way of doing it but this is very time consuming. It is possible if you only want to protect your own inbox but again, spam is getting on your system and you spend time in deleting it.

We use a very simple technique now and have implemented it just before delivery of emails towards the mailboxes. I can’t go too deep into details, spammers might read this also, but it keeps the emails with images for reviewing and also works with whitelisting.