Fix "421 4.7.0 (Outlook)": Outlook temporarily deferred your mail for reputation

Bouncing with 421 4.7.0 (Outlook)? It means Outlook temporarily deferred your mail for reputation.

How to fix it

Slow down, warm your IP, and register with Microsoft SNDS/JMRP.

Prevent it

• Authenticate with SPF, DKIM and DMARC.
• Send from a dedicated 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 a REST API and an AI watching deliverability — and it welcomes senders SES, SendGrid and Mailchimp reject, from $8. Run the free checker or see plans.