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📄️ Overview

E-mail filtering is a crucial part of any modern mail server system. Filtering serves various purposes including the management of incoming and outgoing messages, blocking or flagging spam, removing viruses from attachments, automating organization of messages, and implementing various user-defined rules for message handling. Effective filtering is essential not only for the organization and security of the mail system but also for user productivity and the overall quality of the email experience.

📄️ Milter filters

Milter, or "mail filter", is an extension to mail servers based on the Sendmail protocol. Milters allow third-party software to access mail messages as they are being processed in order to filter, modify, or annotate them. By using Milters, a mail server can utilize a variety of functionalities such as spam filtering, virus scanning, and other types of mail processing, beyond what is built into the mail server itself. Milters operate at the SMTP protocol level, which means they have access to both the SMTP envelope and the message contents.


DNS block lists (DNSBLs) are databases of IP addresses and domain names that are associated with malicious or unwanted behavior on the Internet. They are used as a tool for blocking incoming email from known spam sources. Stalwart SMTP can be configured to use DNSBLs to check the IP address and domain name of the sender of an incoming email, and if they are listed in one of the DNSBLs, the email is rejected. DNSBLs are maintained by various organizations and are updated in real-time to reflect the latest information about spam and malicious activity on the Internet.