Fix error codes
Got a bounce? Find your exact code and fix it
Every code here is verified against the mailbox provider's own documentation — no guessing at what a bounce means.
Gmail
550 5.7.26
Gmail 550 5.7.26 — "This mail is unauthenticated"
Gmail's 550 5.7.26 bounce means your message failed both SPF and DKIM. Here's exactly what Gmail checks and the DNS records that fix it.
421 4.7.0
Gmail 421 4.7.0 — temporary deferral, usually a reputation or rate issue
A Gmail 421 4.7.0 response is a temporary deferral, not a hard rejection — it usually means sending too fast, too much, or with weak sender reputation. Here's how to diagnose and fix it.
550 5.7.25
Gmail 550 5.7.25 — your sending IP has no PTR record
Gmail's 550 5.7.25 bounce means the IP that delivered your message has no valid reverse-DNS (PTR) record. Here's whose problem it is and how to fix it.
550 5.4.5
Gmail 550 5.4.5 — daily sending quota exceeded
A 550 5.4.5 bounce means the Google account sending the mail hit its daily limit. Here's how the limits work, how long the block lasts, and the right long-term fix.
452 4.2.2
Gmail 452 4.2.2 — the recipient's mailbox is over quota
A 452 4.2.2 response means the recipient's Gmail account is full. What it means for one-off mail, and what repeated 4.2.2s should trigger in your list hygiene.
Microsoft 365 / Outlook
550 5.7.1
Microsoft 365 error 550 5.7.1 — message blocked by a security policy
A 550 5.7.1 non-delivery report from Exchange Online usually means a security or transport rule — either yours or the recipient's — is blocking the message. Here's how to find and fix it.
550 5.7.23
Microsoft 365 error 5.7.23 — SPF validation failed
NDR 5.7.23 in Exchange Online most likely points to an SPF configuration problem in your Microsoft 365 tenant. Here's the exact fix.
550 5.7.515
Outlook.com error 550 5.7.515 — sender domain authentication failed
550 5.7.515 from Outlook.com means the message's From-header domain failed authentication under Outlook's sender requirements. Here's what to check.
550 5.1.10
Microsoft 365 error 550 5.1.10 — recipient not found (often backscatter)
550 5.1.10 in Exchange Online means the recipient address doesn't exist — and if you never sent the original message, it's likely backscatter from a spoofed From address.
554 5.2.2
Microsoft 365 error 554 5.2.2 — mailbox full (QuotaExceeded)
Exchange Online returns 554 5.2.2 when the recipient's mailbox is over its storage quota. What senders, recipients, and admins should each do about it.
Generic SMTP
451 4.7.0 (Temperror)
451 4.7.0 Temperror — a temporary DNS lookup failure during authentication
A Temperror during SPF or DKIM evaluation means a DNS lookup timed out or failed temporarily while the receiver was checking your authentication — usually resolves on retry.
550 5.1.1
550 5.1.1 User unknown — the recipient address does not exist
SMTP 550 5.1.1 is a permanent recipient-address failure. Learn how to verify the mailbox, handle aliases, and stop repeat hard bounces safely.
552 5.3.4
552 5.3.4 Message too large — reduce the email size
SMTP 552 5.3.4 means the message exceeded a receiver or gateway size limit. Reduce attachments safely and account for encoding overhead before resending.
554 5.7.1
554 5.7.1 Delivery not authorized — read the policy reason
SMTP 554 5.7.1 is a broad permanent policy rejection. Use the surrounding diagnostic to distinguish authentication, relay, reputation, and content causes.
550 relay not permitted
"Relay not permitted" / "Relay access denied" — who's refusing and why
A relay-denied bounce means an SMTP server refused to forward mail it isn't responsible for. The fix is almost always authentication or a wrong server setting — here's how to tell which.