Fix "550 5.7.133": Outlook rejected the message as unauthenticated

Bouncing with 550 5.7.133? It means Outlook rejected the message as unauthenticated.

How to fix it

Set up SPF, DKIM and DMARC alignment for Microsoft.

Prevent it long-term

• Authenticate with SPF, DKIM and DMARC.
• Send from a dedicated, reputation-clean IP.
• Keep bounce and complaint rates low.

How Rainmail helps

Rainmail sends from your own domain on a dedicated, isolated IP with SPF, DKIM, DMARC, reverse-DNS and managed warm-up handled for you, plus an AI that watches your list health and deliverability — and it welcomes the senders Amazon SES, SendGrid and Mailchimp reject, from $8. Run the free 10-second checker or see plans.