установка greylist postfix ubuntu

For example, the following chart shows the number of spams caught by SpamAssassin on daily basis, on my mail server serving 3 real users. At the beginning of August I set up greylisting, and the number of spams per day dropped dramatically. That means I only need to run one SpamAssassin instance (which is huge at around 30Mb RSS per process), and it does not need to work that hard analysing incoming emails.

As most spams are sent out by bots/dumb SMTP clients, they do not bother to try again after the SMTP server replies with temporary failure, where a properly implemented SMTP MTA would continue to re-try for a certain amount of time. It effectively blocks out majority of spams without actually scanning through their contents.

In name, as well as operation, greylisting is related to whitelisting and blacklisting. What happen is that each time a given mailbox receives an email from an unknown contact (ip), that mail is rejected with a “try again later”-message (This happens at the SMTP layer and is transparent to the end user). This, in the short run, means that all mail gets delayed at least until the sender tries again – but this is where spam loses out! Most spam is not sent out using RFC compliant MTAs; the spamming software will not try again later.

What makes greylisting different from other filter/classifier based defense, like and ? From website:

This article was written after successfully implementing greylisting on using , on a memory-restrained VPS running . Hopefully it will be useful to those who are thinking of implementing a similar solution.

is a relatively new method to use against spams, and its principle is very different from traditional content filtering/content classifying strategy. The differences make it very effective in stopping spams currently, utilising relatively little CPU time and has small memory footage. It by no means is a replacement for fiter/classifier based spam protections, but can be easily deployed as first level of defense to reduce the CPU/memory demand of your spam filters.

If you have an email address for a while, you’ll know that “spams” are almost inevitable. Once the email address has been used, spams will find their way there sooner or later. Combating email spams has also become one of the most researched topics these days.

Greylisting Spams with Postfix + Gld « HostingFu