In today's Internet environment, the spam issue cannot be eliminated 100%. It is a new problem, for which no conventional solutions have yet been designed. To address this increasing issue, many solutions for the stages before and after accepting mail messages were designed, to ensure most of the spam messages do not get to reach users’ mailboxes.

This article will focus on the most relevant ones, with a high degree of usability.

Introduction There are several methods that can be used to make a gross selection of the incoming messages; out of them, we can separate two principles of spam filtering. We will thus focus on the available mechanisms and methods, and consider pre-accepting a message filtering as the gross filter, and post accepting the message as the fine filter.

Gross filtering usually consists in checks that the connection complies with certain conditions before it is accepted by the server; also note that this type of filtering may not exist at all in some mail servers, depending on their configuration and implementation. Another aspect that needs to be taken into consideration is that, in certain configurations, these checks may not be made by the actual SMTP server, but by third party utilities and tools, or by intermediary tools that intercept or mitigate the communication between your server and the connecting host.