Fix "550 5.1.1": The recipient mailbox does not exist (user unknown)

If your email is bouncing with 550 5.1.1, it means the recipient mailbox does not exist (user unknown). Here is exactly what causes it and how to fix it.

What 550 5.1.1 means

550 5.1.1 means the address is invalid or was deleted. A high rate of these hurts your reputation, so treat them as permanent and remove them immediately.

How to fix 550 5.1.1

• Suppress the address permanently after a 5.1.1.
• Validate new lists before sending to cut invalid addresses.
• Keep bounce rate under ~2% to protect sender reputation.

How Rainmail helps

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