Email server guard defends against spam through SMTP

As enterprise users use their mailboxes for a longer period of time, they will encounter more and more spam problems. They will not only spend valuable time processing spam every day, but also consume a large amount of Enterprise mailbox space, and occupy a large amount of network bandwidth, and even cause network congestion, resulting in server paralysis. Therefore, when designing an email server, an important part of enterprises is to block spam. Which mainstream defense technologies can be used to restore a clean and stable email system? Currently, there are three main methods to prevent spam: 1. IP domain name type; 2. Content Filtering type; 3. SMTP technical specifications.

Today we will talk about SMTP, Which is unfamiliar to ordinary people. SMTP (Simple Mail Transfer Protocol) is a Simple Mail Transfer Protocol. It is a set of rules used to send Mail from the source address to the destination address, which controls the Transfer mode of Mail. The so-called "SMTP Technical Specification" is to use ESMTP (Extended SMTP protocol) for verification, access the specified destination address, in order to filter out spam.

Next, we recommend five methods to defend against spam through SMTP:

-Grey-Listing: indicates a gray list. This technique uses the SMTP protocol to indicate that the receiving server is temporarily unavailable, the sender is obligated to resend the mail after a period of time, while the spam mailing software usually does not resend the mail. This method has some disadvantages, causing the recipient to delay receiving the mail.

-SMTP Banner information fragmentation: this technique intentionally sends Banner information in segments, which may result in poor SMTP sending program being unable to judge Banner information and cause sending failure. A famous foreign email server adopts this method.

-SMTP Banner information delay: the SMTP protocol stipulates that the waiting time for the Banner can be up to 15 minutes. Many spam software will give up sending if they are impatient.

-Smtp ehlo/HELO Response Check: This method uses the SMTP protocol to check that the host domain name in the EHLO command must be resolved to the sender host, many mailing software programs cannot make correct judgments on their domain names. In this case, they cannot send spam.

-SMTP command sequence restriction: the SMTP protocol specifies that the sender can issue EHLO after the server sends a Banner. Many email servers deliberately delay sending the SMTP Banner, however, many spam sending software directly sends EHLO without waiting for the Banner, which also serves as a basis for being judged as spam.