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Greylisting is an anti-spam technique where mail servers temporarily reject emails from unknown senders, returning a "try again later" response. Unlike blocklisting, greylisting is a temporary measure that tests whether the sending server follows proper email protocols. Legitimate mail servers will retry delivery after the delay, while spammers typically move on to their next target.
Greylisting affects your email delivery timing and can delay time-sensitive messages. If you send from a properly configured mail server, greylisting causes only a brief initial delay for each new recipient server. However, misconfigured servers that don't retry properly will have emails permanently rejected. Understanding greylisting helps you troubleshoot delivery delays, configure retry policies correctly, and set appropriate expectations for first-contact email delivery times.
When an email arrives from an unfamiliar sender, the receiving server records three pieces of information: the sender's IP address, the sender's email address, and the recipient's email address. The server then rejects the message with a temporary error code (typically 451), instructing the sender to try again later. Legitimate mail servers are programmed to retry failed deliveries after a short delay (usually 5-30 minutes). Spam servers, optimized for volume over reliability, rarely retry and simply move on. After a successful retry, the sender is added to a whitelist, and future emails pass through without delay.
Initial delays range from 5 minutes to several hours, depending on the receiving server's greylisting policy and your mail server's retry interval. Most delays are under 30 minutes. Once you've successfully delivered to a recipient, future emails typically arrive without delay.
No. Greylisting is temporary and affects only first-time senders until they prove legitimacy by retrying. Blocklisting is a more permanent measure where your IP or domain is flagged as a spam source. Greylisted emails will eventually deliver; blocklisted emails may be permanently rejected.
Yes, primarily through delivery delays. Time-sensitive campaigns may reach some recipients hours later than others. Using reputable email service providers minimizes this issue, as their IPs are often whitelisted. For new sending domains, expect some initial delays as you build reputation.
Check your mail server logs for 451 response codes with messages like "try again later" or "greylisted." If emails are delayed but eventually deliver on retry, greylisting is likely the cause. Consistent failures without delivery suggest a different issue, possibly blocklisting.
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