Fix "421 4.2.1": The service is unavailable or shutting down

Bouncing with 421 4.2.1? It means the service is unavailable or shutting down.

How to fix it

Retry later; the receiving server is temporarily offline.

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.