Check reverse DNS and HELO
Reverse DNS and HELO/EHLO are the identity signals a receiving server sees before it ever evaluates your content. They matter most on dedicated sending IPs and self-managed mail servers, where a missing PTR or generic HELO name can make otherwise authenticated mail look suspicious.
Why mailbox providers enforce this
Receivers use reverse DNS to check whether an IP has a meaningful hostname, and HELO/EHLO to identify the sending mail server during SMTP. A dedicated IP with no PTR, a PTR that does not resolve forward, or a HELO name like localhost suggests low-quality or misconfigured infrastructure. Shared providers often manage this for you; dedicated infrastructure makes it your responsibility.
How to fix it
- Confirm the sending IP has a PTR record with a real hostname, not a generic cloud-provider default.
- Confirm that hostname resolves forward to the same sending IP where possible.
- Set HELO/EHLO to a fully qualified domain name that resolves and matches your mail infrastructure.
- Use a stable mail hostname such as mail.yourdomain or mta1.yourdomain instead of localhost, an IP literal or a random instance name.
- If you send through Gmail, Outlook, SES or another managed relay, do not chase shared relay PTRs as if they were yours; focus on domain authentication and reputation.
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FAQ
Does reverse DNS have to match my From domain?
Not exactly. It should identify legitimate mail infrastructure and resolve cleanly. Exact brand-domain matching is less important than a stable, non-generic, forward-confirmed hostname.
Can bad HELO cause spam placement?
Yes, especially on dedicated or self-hosted mail. Some receivers reject malformed HELO names outright; others treat generic or non-resolving HELO as a negative reputation signal.
Related
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