What is Greylisting?
Greylisting is a spam prevention technique that temporarily rejects emails from unknown senders, expecting legitimate mail servers to retry delivery while spammers typically don't.
Greylisting is an anti-spam technique where a receiving mail server temporarily rejects emails from unknown senders with a "try again later" response (4xx error). The server records the sender IP, recipient, and sender address—this combination is called a "triplet."
When the sending server retries (as legitimate servers do), the recipient's server recognizes the triplet and accepts the message. Most greylisting systems require a retry after a delay (typically 1-15 minutes) but before a timeout (typically 4 hours).
Why it works: Legitimate mail servers are configured to retry delivery when they get a temporary failure. Many spam systems, designed for high-volume sending, don't bother retrying—they move on to the next target. Greylisting exploits this behavioral difference.
The downside: Greylisting delays legitimate email, sometimes significantly. This is particularly problematic for time-sensitive transactional emails like password resets or two-factor authentication codes.
Why Greylisting Matters
Greylisting causes deliverability issues that can be confusing to diagnose. Your email isn't rejected—it's delayed. For marketing emails, a 15-minute delay might be fine. For a password reset or authentication code, that delay makes your application feel broken. Understanding greylisting helps you troubleshoot mysterious delays and explain them to users. It's also why reliable retry logic in your sending infrastructure matters.
How Ark Handles Greylisting
Ark's infrastructure properly handles greylisting with intelligent retry logic. When we receive a temporary rejection, we automatically retry with appropriate backoff timing until the message is accepted or the receiving server gives a permanent failure. Our transactional focus means we understand the urgency—we retry quickly while respecting the receiving server's policies.
Frequently Asked Questions
How is greylisting different from blacklisting?
Blacklisting permanently blocks emails—they're rejected outright. Greylisting temporarily delays emails—they're accepted after the sender retries. Greylisting is a speed bump, not a roadblock.
Why are my emails delayed by 15+ minutes?
Greylisting is a common cause. The recipient's server temporarily rejected your email, and your server had to retry after the required delay. Check if delays are consistent and concentrated on certain recipient domains—that suggests greylisting.
Can I bypass greylisting?
You can't force your way through, but recipients can whitelist your domain or IP to bypass greylisting. If you have B2B customers experiencing delays, ask their IT team to add your sending IPs to their allowlist.
Does greylisting affect all email providers?
No. Large providers like Gmail and Microsoft generally don't use traditional greylisting because their spam filtering is sophisticated enough without it. Greylisting is more common with smaller providers, enterprise mail servers, and security appliances.
Should I enable greylisting on my mail server?
It depends on your needs. Greylisting reduces spam with minimal resources but delays legitimate email. If you receive time-sensitive emails, the delays may be unacceptable. Many organizations now prefer content filtering and reputation systems over greylisting.
Why do large senders sometimes fail greylisting?
Large providers (like Microsoft 365) may use different IPs for the initial send and retry, which confuses greylisting systems expecting the same IP. This is a known issue that causes legitimate enterprise email to be delayed or fail.
Related Terms
Soft Bounce
A soft bounce is a temporary email delivery failure caused by issues like a full mailbox, server dow...
Email Deliverability
Email deliverability is the ability to successfully deliver emails to recipients' inboxes rather tha...
IP Warmup
IP warmup is the process of gradually increasing email volume from a new IP address to build sender ...
Allowlist
An allowlist (previously called whitelist) is a list of approved senders whose emails bypass spam fi...
Ready to improve your email deliverability?
Ark handles greylisting and more automatically. Start sending in 5 minutes.