What is Email Greylisting?

Email greylisting is a method of protecting email users from spam. A Mail Transfer Agent (MTA) that uses greylisting blocks a suspicious email with a “temporary rejected” error. In such a situation, a legitimate SMTP server makes multiple attempts to resend a delayed email until it is finally accepted. Servers for mass email spam, on the contrary, usually do not have such sophisticated features.

How Email Greylisting Works

SMTP rules provide that a greylisting server filters unknown email servers and gives a “temporarily rejection” back to the sending server. Legitimate SMTP servers retry sending after as much as 15-minute delay by default, on and on.

If a greylisting filter considers an email suspicious but lets it through after retrying the delivery, the next emails are going through without delays. This is because the greylisting server remembers the sender’s name and other details and whitelists it without any additional settings from a user.

Another positive aspect of greylisting is that for MTAs, sending a temporary error is a cost-effective measure since it doesn’t require much CPU (central processor) power and memory for its performance, unlike most of the other spam filtering programs.

Unfortunately, in cases where the sending server is poorly set up, the delay time may be far more than 15 minutes by default, which might irritate recipients if they have gotten used to the instantaneous nature of emails. Another disadvantage is that junk mailers adjust to greylisting and queue spam emails for redelivery, which weakens this anti-spam method. Anyway, the delay buys some time for other systems to identify the suspicious sending server as a source of spam.