Error codes

What these pages are: the receiver's own rejection text, what it means, and how to check and fix your records.

• 550 5.7.26 Gmail: "This email has been blocked because the sender is unauthenticated"
• 550 5.7.27 Gmail: "This message was blocked because it didn't pass SPF authentication"
• 550 5.7.30 Gmail: "This message was blocked because it didn't pass DKIM authentication"
• 550 5.7.40 Gmail: "the sending domain doesn't have a DMARC record"
• 421 4.7.28 Gmail: "Gmail has detected an unusual rate of email"
• 550 5.7.515 Microsoft: "Access denied, sending domain does not meet the required authentication level"
• Gmail: "This message is suspicious due to the very low reputation of the sending domain"