Fix "421 4.3.2": The service is not available, closing channel

Bouncing with 421 4.3.2? It means the service is not available, closing channel.

How to fix it

Retry later; the receiver is overloaded or throttling.

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.