What does your bounce message actually mean?
A bounce message carries an SMTP reply such as 550 5.7.26 or 421 4.7.28. The first digit says whether it is permanent (5) or temporary (4); the rest says why. Paste your bounce below to decode it, then follow the fix, with every message quoted from Google or Microsoft’s own documentation.
All bounce codes by cause
Authentication (SPF, DKIM, DMARC, PTR)
- 5.7.26Unauthenticated sender (no SPF or DKIM pass)
- 5.7.27SPF authentication failed
- 5.7.30DKIM authentication failed
- 5.7.40No DMARC record, or no DMARC policy
- 5.7.32From: domain not aligned with SPF or DKIM
- 5.7.24Suspicious entries in your SPF record
- 5.7.25Sending IP has no reverse DNS (PTR) record
- 5.7.23Rejected for an SPF violation (Microsoft)
- 5.7.509DMARC failed and the policy is reject (Microsoft)
- 5.7.515High-volume sender fails Outlook.com authentication requirements
- 5.7.512Missing or invalid From: header
Reputation, blocks and bans
- 5.7.28Unusual rate of unsolicited mail
- 4.7.0Temporarily deferred: low reputation, PTR or TLS
- 5.7.1Delivery not authorized, message refused
- 5.7.708Access denied, traffic not accepted from this IP
- 5.7.705Access denied, tenant has exceeded threshold
- 4.7.500+Access denied, please try again later
- 5.7.606+Access denied, banned sending IP
- 5.7.511Access denied, banned sender (IP)
- 5.7.501Access denied, spam abuse detected (banned sender)
- 5.7.513Your IP is on the recipient’s own block list
Rate limits and throttling
- 4.2.1Recipient is receiving mail too quickly
- 5.2.121Recipient’s hourly receive limit exceeded
- 5.2.2Mailbox full, or submission quota exceeded
- 5.1.90Daily recipient limit reached
- 5.7.233Tenant exceeded its daily external recipient limit
- 5.7.236onmicrosoft.com domain hit 100 external recipients
- 4.7.1Temporarily refused (often greylisting)
Recipient address and mailbox problems
Your account, tenant or relay setup
TLS and delivery transport
Questions
Bounce code questions
What is the difference between a 4xx and a 5xx bounce?+
A reply starting with 4 (such as 421 4.7.28) is a temporary failure: the sending server keeps the message and retries for a while, usually up to a few days. A reply starting with 5 (such as 550 5.7.26) is permanent: the message will not be retried, and resending unchanged will fail again.
What is an enhanced status code?+
It is the three-part code such as 5.7.26 that follows the three-digit SMTP reply. RFC 3463 defines the structure: the first digit is the class (4 temporary, 5 permanent), the second is the subject (1 addressing, 2 mailbox, 4 routing, 7 security and policy), and the third is the specific detail. Providers add their own detail numbers on top.
Where do I find the code in a bounce message?+
Open the bounce (it usually comes from "Mail Delivery Subsystem" or "Microsoft Outlook") and look near the top for the remote server’s reply. Gmail bounces show lines like "550-5.7.26"; Microsoft 365 bounces show "Remote server returned …" followed by the code. Pasting the whole bounce into the decoder above works too.
Is my bounce message uploaded when I use the decoder?+
No. The decoder runs entirely in your browser. The text you paste is never sent to WarmHawk or anyone else.
Where do these error messages come from?+
Every message on these pages is quoted from the provider’s own documentation: Google’s Gmail SMTP errors and codes, Microsoft Learn’s Exchange Online NDR reference, Microsoft Support for Outlook.com, and RFC 3463 for the standard meanings. They were last checked on 2026-09-29, and each page links its sources.