Emails delayed: greylisting and 4xx temporary errors

What it means

When a receiving server answers with a code starting with 4, it means "not now, try again later". Your server keeps the email and retries automatically. Common cases:

• Greylisting: the receiver refuses the first attempt from a sender it does not know, for example with 451 4.7.1 Greylisted, please try again later. Real mail servers retry after a few minutes; most spam robots never do. On the second try, the email is accepted.
• Rate limiting: you send too many emails too fast, and the receiver slows you down, for example Gmail's 421 4.7.28 or Yahoo's 421 4.7.0 [TSS04].
• Temporary problems: a full mailbox (452 4.2.2), a busy or broken server, a DNS lookup that failed.

Why it matters

• Greylisting alone is harmless: the delay is usually 5 to 30 minutes, and it happens only the first time.
• Repeated 421 rate limits are a warning about your reputation. If you ignore them, they can become permanent rejections.
• If your server does not retry, or retries badly, the email is lost.

How to check

• Read the bounce or delay notice. Many servers send "Delivery delayed" after a few hours. The line with the 4xx code explains the reason.
• Look at your server logs if you run your own server. Lines such as status=deferred show each delayed attempt.
• Send a test email from the home page (/) to your Delivmail test address. If it arrives quickly and the report is clean (SPF, DKIM, DMARC, reverse DNS, no blacklist), the delay comes from the receiver's policy, not from your setup.
• Check your IP and domain at /blacklists: some receivers slow down listed senders.

How to fix it

Greylisting

1. Do nothing special. Let your server retry. Do not resend the email by hand, which creates duplicates.
2. Keep the same sending IP for retries. Big providers with many IPs can fail greylisting repeatedly; they usually handle it, but if you run several servers, retry from the same one.
3. If the delay is a problem for important emails (password resets, codes), ask the recipient's admin to allow your domain or IP.

Rate limits

1. Slow down. Send in smaller batches, spread over time.
2. Warm up new IPs and domains gradually instead of sending your whole list on day one.
3. Improve your reputation: authenticate with SPF, DKIM and DMARC, remove bounces, and make unsubscribing easy.

Your own server

• Check that it retries for several days (the usual default is about 5 days) with growing intervals.
• Make sure it has a valid reverse DNS (PTR) and a correct HELO name, which reduces delays.

Check that it is fixed

After a first greylisted delivery, the next emails to the same receiver should arrive without delay. For rate limits, watch your logs: deferred messages should become rare as you slow down and your reputation improves. Run a Delivmail test regularly to confirm your setup stays clean.