Spam continues to be the bane of email productivity and is something I’ve covered in “Tech Support” multiple times. From my very first Linux Magazine “Take back your INBOX” article in December 2003 to “Stop More Spam” in November 2006, I hope I’ve helped you keep your sanity — at least when it comes to reading your email. Spammers are increasingly clever and are always adding new tactics to their arsenal, so it’s important that you do the same. Today I’d like to introduce you to greylisting, a method I’ve found extremely effective in decreasing spam.
An MTA that employs greylisting will temporarily reject email it does not recognize. This is typically based on the IP address of the connecting host and the envelope sender/recipient, but may vary slightly in some implementations. A properly configured MTA will requeue the mail based on this temporary rejection and try again in a short amount of time (typically between 5-15 minutes).
Since most spammers are attempting to send as much email as possible as quickly as possible, they will typically not retry. Even if they do retry, the small delay means there is an increased chance the spam content will be in the RBL’s and distributed signature systems you use for spam detection. It should be noted that there is a downside to using greylisting. By definition it adds a small delay to some email, which can cause consternation with users who expect email to be real time.
Additionally,…
Please log in to view this content.
Not Yet a Member?
Register with LinuxMagazine.com and get free access to the entire archive, including:
