Understanding Common Email Bounce Messages and Errors

Sometimes you may be checking logs and see emails sitting in the queue, waiting to be retired or perhaps you got a bounce back – below are common retry, error or bounce messages you may see in your email logs. Some are temporary errors on the remote server, while others might indicate configuration issues. If an automatic retry doesn’t succeed, these emails will bounce back to the sender.

Here’s a breakdown of what they mean and what you can do.

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Common Bounce Messages Explained

1. SORBS RBL Error

450 Requested action not taken:front1, XACowACXn6_A4w1p4Zr3AA–.4596S3, dnsbl.sorbs.net, please try again

Explanation: The SORBS spam list no longer exists (it closed down on June 5, 2024). When you see this error, it means the remote receiving system is still trying to use this non-existent RBL. A retry will be attempted, but it may not succeed if the receiving server’s configuration is not updated.

2. Spamhaus RBL Error (Open Resolver)

550-“JunkMail rejected – relay2-bounce-e.mailbaby.net [XXX]:45624 is in an RBL: Error: open resolver; https://check.spamhaus.org/returnc/pub/172.70.204.5/”

550 5.7.1 Service unavailable; client [XX] blocked using zen.spamhaus.org

Explanation: Spamhaus does not allow queries to its free RBL infrastructure from public/open DNS resolvers (like Google’s 8.8.8.8 or Cloudflare’s 1.1.1.1). The receiving server is using one of these resolvers to check for spam, which Spamhaus blocks, causing the email to be rejected. The remote recipieint is likely not getting much of their email, if it has been happening long term you may need to find another way to contact them.

3. Gmail Rate Limiting (SRS/Forwarding)

421-4.7.28 Gmail has detected an unusual rate of unsolicited mail. To protect our users from spam, mail has been temporarily rate limited.

Explanation: This often happens with SRS-forwarded emails to Gmail, particularly with automated DMARC reports that look similar to this:

From: [email protected]

Subject: Report domain: XXX Submitter: google.com Report-ID: XXX

It appears Gmail may rate-limit bulk-forwarded messages that have similar content or subjects. These emails are typically delayed but do eventually arrive as they are retried.

Options:

• Consider if you actively check these specific DMARC reports.
• Consider not forwarding these reports to a Gmail address.
• Use a dedicated DMARC monitoring service (like MXToolbox) to ingest and manage these reports for you.

4. Yahoo DNS Deferral (Unresolvable From Domain)

451 Message temporarily deferred due to unresolvable RFC.5321 from domain. See https://senders.yahooinc.com/error-codes#unresolvable-from-domain

Explanation: This is common with Yahoo, which is stricter about DNS compliance than many other providers. It often happens if you have parent and child nameserver mismatches. Even if your DNS appears to work elsewhere, an underlying error exists.

Solution: Check your domain with a tool like intodns.com. You may see errors such as:

Missing nameservers reported by parent – FAIL: The following nameservers are listed at your nameservers as nameservers for your domain, but are not listed at the parent nameservers…

Missing nameservers reported by your nameservers – ERROR: One or more of the nameservers listed at the parent servers are not listed as NS records at your nameservers.

5. Mimecast Greylisting

451 Internal resource temporarily unavailable – https://community.mimecast.com/docs/DOC-1369#451

Explanation: This is a standard greylisting message from Mimecast. The email will be retried automatically and should be accepted after a short delay.

6. Network or DNS Error (Typo)

Network error: Network error when connecting to MX server otmail.com for otmail.com: Connection timed out

DNS Error: DNS error occurred while resolving the Mail Exchange (MX) server for the specified domain DNS server returned general failure

Explanation: Most often, this is caused by a simple misspelled domain (e.g., otmail.com instead of hotmail.com) when an email address was entered manually.

Caution: While one-off spelling mistakes aren’t usually a major problem, be careful, as some misspelled domains are set up as spam traps. This type of error is more concerning if it appears many times from a mailing list that claims to be double opt-in. Common domain misspellings are a sure sign of a bad or purchased mailing list.

7. Receiving Mail Too Quickly

450 User is receiving mail too quickly tnmpmscs

Explanation: The remote email address is receiving too much email at once. While the recipient may just be very popular, you should be cautious if you see this error frequently from a sign-up form (like new user creation).

Warning: Your form is likely being abused by bots to spam or mailbomb the recipient. All web forms that send email should be protected by some type of verification, such as a CAPTCHA, Turnstile, or another anti-bot solution.

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Protecting Your Outbound Email Reputation

Mailbaby is designed to protect outbound email systems by acting as an email smart host. This is done by:

• Forcing better email compliance
• Identifying non-double opt-in lists
• Detecting compromised accounts
• Preventing outbound spam
• Monitoring SMTP responses and automatically making decisions based on bounce rates and hard bounces

Add Mailbaby to protect your outbound queue reputation and your clients’ domain reputations.