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.
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.