Fix "504 5.5.2": The server requires a valid HELO/EHLO domain

Getting 504 5.5.2? It means the server requires a valid HELO/EHLO domain.

How to fix it

Configure your mail server to send a proper fully-qualified HELO name.

Prevent it

• Authenticate with SPF, DKIM and DMARC.
• Use 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, 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.