SMTP Error Codes Explained (With a Lookup Table)

What SMTP bounce messages mean and how to fix the cause.

550 5.7.26: Unauthenticated Email Is Not Accepted (Gmail)Gmail's 550 5.7.26 bounce means your message failed authentication: no passing SPF or DKIM, an SPF hard fail, or a failed DMARC check under p=reject.

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