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IMAP and Spam Filtering

If you store your mailboxes on a remote IMAP server, it is a waste of bandwidth, time, and resources (both human & machine resources) to do spam filtering on your local system. The best place to do spam filtering is on your incoming email server. Here's why:
  1. Your email system administrator can keep the spam filters up to date. This includes both SMTP-level and delivery-level filters. Here are some details about these.
    • SMTP-Level Filters Very robust spam filtering can be done during the SMTP conversation before the message is accepted by your incoming email server. If a message is not accepted, the SMTP protocol will send an SMTP undeliverable-mail message to the sender that states that the message was blocked and why it was blocked. This is by far the best place to filter incoming spam.
    • Delivery-Level Filters Once a message is accepted by the SMTP server, there are very good constantly-changing tools that can determine how spammy a message is.
  2. Spammy messages can be kept out of your default INBOX and instead delivered to an IMAP-accessible "MaybeSpam" mailbox.
  3. There is no need to download the spammy messages to your local system.
  4. You are not locked in to any particular email client or any particular computer, and you can, instead, manage your spam and your spam filters from many different systems.
If you use a desktop email client that has built-in spam filtering, I recommend that you turn off desktop spam filtering and let your IMAP service provider filter your spam. One of the keys to successful spam filtering is being able to greenlist false positives, i.e., greenlist any non-spam message that was blocked at the SMTP level or that was delivered to your MaybeSpam mailbox.

Make sure that you get an IMAP service provider that makes it easy to . . .
  • view reports of messages sent to you that were blocked at the SMTP level.
  • set up your own SMTP-level greenlists.
  • sort (order) your MaybeSpam messages by spam score (aka "spamminess" or "spam probability"). This makes it much easier to find false positives, i.e., non-spam messages that were delivered to your MaybeSpam mailbox.
  • set up your own delivery-level greenlists so you can minimize your false positives.
  • specify your own spam-score threshold for what you consider to be MaybeSpam.
The more that people demand these features from their email service providers, the more likely it is that we will all be able to have control over our own email -- both our spam email and our non-spam email.
posted Thursday, May 25, 2006   |   permalink to this page

 

 

Comments

Nancy, thanks for this once again concise and comprehensive post.
above comment posted by Blogger rattus on 26 May, 2006   |   permalink to this comment
 

 

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