Contents
- What is greylisting?
- How does greylisting work?
- Advantages and disadvantages of greylisting
- How to avoid being greylisted?
What is greylisting?
Greylisting, or the creation of grey lists, is an automated mechanism for filtering unwanted email (spam) at the SMTP protocol level. This technology is based on the observation that spammers often do not comply with the requirements set out in the protocols. For example, when a server receiving an email refuses to accept it, compliant mail servers of the senders will attempt to resend the message. In contrast, spammers typically do not make such attempts, which reveals them.
How does greylisting work?
The term "greylisting" comes from the English word "greylist." It serves as an intermediate solution between blacklists and whitelists in mailing systems. When a sender tries to send the first email to your mailbox, the server using greylisting technology simulates unavailability and refuses to accept that message. At this point, the sender's address is added to the "grey" list.
If the sender attempts to resend, the email will be successfully delivered. From this moment on, the address will be remembered as allowed, and all subsequent messages from that sender will be accepted without delays.
Advantages and disadvantages of greylisting
Although greylisting is not a universal solution for combating spam, in practice it can filter out about 90% of unwanted emails. This technology has several positive aspects:
- Does not require user intervention;
- Uses minimal resources from the server system;
- Avoids loss of legitimate emails during filtering;
- Allows the creation of a whitelist of servers, enabling the receipt of emails from important senders without delays.
However, the method also has its drawbacks. The main one is the delay in the delivery of most messages (approximately 30 minutes), which can be critical for urgent correspondence. This only applies to the first email in the correspondence; all subsequent ones will arrive quickly. To reduce delays, reliable senders can be added to the whitelist.
Additionally, greylisting does not protect against spam sent from a fully functional mail server. Spammers are also evolving and may start sending emails that comply with all protocol requirements.
How to avoid being greylisted?
To avoid being placed on "grey lists," it is important to consider several recommendations:
- Care for the reputation of the IP address;
- Use a reliable domain;
- Provide an unsubscribe option at any time;
- Sign emails with a real name.
It is also worth remembering that mailing services have less strict rules regarding "grey lists." Using a specialized service can speed up the delivery of emails to recipients. Greylisting will be particularly useful for small companies looking to shield themselves from spam. With low costs and ease of implementation, this technology effectively cuts out email junk.