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

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 a 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 (main processor) power and memory for its performance unlike most of the other spam filtering programs.

Unfortunately, in case the sending server is poorly set up, the delay time may be far more than 15 mins by default, which might irritate the recipients, got used to the instantaneous nature of emails. The other 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 spamming.

How to avoid email greylisting

  • Take care of your IP-address reputation. SendPulse ensures a good IP-address reputation, minimizing the chances to get greylisted.
  • Use a reliable domain. In the email address, email domain goes after @ symbol, for example, It takes time to earn a reputation for your email domain.
  • Sign your email with a real sender’s name. A real name instead of something like that —, helps to build trust among your audience and increase open rates. An attractive sender’s name should be like this: Adam at and influence your reputation in the eyes of both the audience and spam filters.
  • Allow instant unsubscribing. Many reasons may cause users’ intention to unsubscribe, but not allowing them to do so leaves no other choice but mark email as spam. If the critical number of subscribers marked your emails as spam, chances are that your IP-address reputation will suffer, and greylisting filters will consider emails from your side as suspicious.
  • Format the headings and the message correctly. The headings and the message itself should be formatted due to RFC 5322 (internet message format) and HTML standards.
  • Avoid using stop-words. Stop words like “Buy,” ”Amazing,” “Free,” “Cheap,” etc., may alarm greylisting filters that an email is not trustworthy. Be extremely careful with emails that are expected to be instantly delivered like welcome, transactional, confirmation, etc.

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