What is greylisting and why doesn't email arrive immediately?

Have you ever noticed that some emails only arrive after several minutes or even hours? There's a good chance you're dealing with greylisting. This antispam technique is used by many mail servers to block spam, but it can also delay legitimate transactional emails. At Lettermint, we make sure your emails arrive as quickly as possible despite greylisting. In this article, we explain what greylisting is and how we handle it.

What is greylisting

Greylisting is a method that some receiving mail servers use to fight spam. It's mostly used by smaller email providers, corporate servers and institutions like universities. Large providers like Gmail and Outlook use it less often. When these servers receive an email from a new or unknown combination of sender and recipient, they may decide to temporarily reject the email. The server then sends back a message asking to try again later.

Why does this work? Spammers send millions of emails and don't have time to retry rejected messages (greylisted). Legitimate mail servers like Lettermint do retry automatically. In the Lettermint dashboard, you'll see this as a "Delayed" status.

How greylisting works exactly

We now know that receiving mail servers can temporarily reject emails. But how do they decide which emails? With greylisting, the server keeps a list with three pieces of information, also called a "triplet":

1. The IP address of the sending server
2. The sender's email address
3. The recipient's email address

When a new combination of these three arrives, the recipient's mail server may decide to temporarily reject the email with the message "please try again later". The recipient's mail server remembers this combination and starts a waiting period of usually 1 to 15 minutes.

After this waiting period, the recipient's mail server will accept emails from the same combination. A successful delivery often puts the sender on a whitelist, so future emails with this combination are accepted immediately. Spammers don't wait this time, which automatically filters out their emails.

Impact of greylisting on email delivery

For regular email traffic or broadcast emails, a delay of a few minutes is often not a problem. But for transactional emails, it's different. A customer requesting a password reset expects it in their inbox within seconds. A 15-minute delay can lead to frustration.

Different providers, different rules

Not all email providers use greylisting equally. Large providers like Gmail and Outlook rarely use it. But smaller providers, corporate servers and private mail servers use it often. Government organizations, educational institutions and companies with strict security requirements especially work with greylisting. The waiting time ranges from a few minutes to an hour.

How Lettermint handles greylisting

Greylisting doesn't have to be a problem for your emails. At Lettermint, we make sure your mail arrives quickly despite these delays, as you can see from our Time to Inbox statistics.

We automatically detect greylisting and know the waiting times for many receiving mail servers. This allows us to retry at exactly the right moment and minimize delays. Additionally, our infrastructure already has a good reputation with mail servers. Since greylisting looks at the sender's IP address and our servers are already known, greylisting happens less often with Lettermint.

Tips to minimize greylisting

While Lettermint handles a lot for you, there are some things you can do yourself to reduce greylisting:

1. Use a consistent sender address. Don't constantly switch between different email addresses. For example, always use the same info@yourdomain.com or hello@yourdomain.com address for transactional emails. Each new combination of sender and recipient has to go through the greylisting process again.
2. Set up proper DNS configuration. Properly configured SPF, DKIM and DMARC creates more trust with receiving servers. This can help you get on whitelists faster.
3. Start gradually with new email addresses. Starting to send from a new email address? Begin with low volumes and gradually build up. This prevents receiving servers from immediately marking you as suspicious.

Conclusion

Greylisting is an antispam technique that unfortunately can also delay legitimate emails. With Lettermint, you don't need to worry about this. Our smart retry attempts and good reputation ensure your emails arrive quickly despite greylisting.

By using a consistent sender address, setting up your DNS correctly, and gradually building up with new email addresses, you minimize the impact even further. This keeps your transactional emails reliable and fast, exactly as your customers expect.