📄️ 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.
📄️ Spam filter
Spam filtering is an essential component of any modern mail server, designed to sift through vast amounts of incoming email to identify and segregate unsolicited or potentially harmful messages. These unsolicited messages, commonly known as spam, not only clutter an individual's inbox but can also pose significant security risks. Efficient spam filtering ensures that genuine, legitimate emails reach their intended recipients, while unwanted or malicious emails are quarantined or discarded.
📄️ Sieve scripting
Sieve is a scripting language used to filter and modify email messages. It provides a flexible and powerful way to manage email messages by automatically filtering, sorting, and transforming them based on a wide range of criteria. Rather than relying on a proprietary DSL, Stalwart SMTP uses Sieve as its default scripting language primarily because it is sufficiently powerful to handle most filtering tasks and is an established internet standard.
📄️ 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.
📄️ Pipes
Stalwart SMTP supports filtering messages using external executable files, referred to as "pipes". Pipes operate by receiving the email message through standard input (stdin), processing or modifying it as required, and then returning the adjusted message via standard output (stdout).