Frequently Asked Questions

What does it mean for HELO/EHLO to match PTR?

For a sending IP, PTR should resolve to a stable FQDN (for example, 203.0.113.10 -> mail1.example.com), and your SMTP server should present that same hostname in EHLO/HELO. The hostname should also resolve forward (A/AAAA) back to the sending IP.

Is an exact HELO = PTR string match required by SMTP standards?

RFC 5321 requires a valid host identity in EHLO/HELO (or an address literal if no name is available), but receivers define local anti-spam policy. In practice, exact and consistent HELO/PTR alignment is the safest choice for deliverability.

What is forward-confirmed reverse DNS (FCrDNS), and do I need it?

FCrDNS means the PTR hostname resolves forward to the same sending IP. Many filters use this as a trust signal, so PTR-only without forward confirmation is weaker.

How many PTR records should a sending IP have?

Use one stable PTR hostname per sending IP whenever possible. Large receivers like Google explicitly recommend a single PTR record per outbound IP.

Can I keep provider-generated PTR names for outbound mail?

You can, but custom hostnames under your domain are usually better for reputation and troubleshooting. Avoid PTR names that look dynamically assigned or generic when possible.

Who controls PTR records, and how do I change them?

PTR is controlled by the owner of the IP block (ISP/cloud/hosting provider), not by your normal forward DNS zone alone. Update reverse DNS in your provider portal or request delegation from the IP owner.

Do IPv6 sending servers need PTR and HELO alignment too?

Yes. Reverse DNS applies to IPv4 and IPv6, and mail providers evaluate both. For IPv6, use an ip6.arpa PTR and keep EHLO/PTR/forward DNS consistent.

How do SPF, DKIM, and DMARC relate to HELO/PTR?

HELO/PTR are connection-level identity signals, while SPF/DKIM/DMARC validate message-level authentication and alignment. You need both: clean server identity plus correctly aligned domain authentication.

When does SPF use the HELO identity?

When MAIL FROM is empty (common for bounces), SPF checks the HELO/EHLO domain instead of the envelope sender domain. That makes a valid, DNS-resolvable HELO hostname important.

What HELO hostname format is best practice?

Use a fully qualified domain name that you control, with standard host-label syntax, and keep it stable per sending system. Avoid localhost, bare domains, or hostnames that do not resolve cleanly.

What errors usually indicate PTR/HELO issues?

Common signs include SMTP rejections mentioning missing PTR, reverse DNS mismatch, or failed sender identity checks (for example, Gmail 5.7.25 class errors). If seen, verify PTR, forward DNS, and EHLO name together.

What is a practical HELO/PTR checklist before going live?

For each outbound IP: set one PTR, set EHLO to that FQDN, ensure A/AAAA maps back to the same IP, publish SPF/DKIM/DMARC, and test from the exact production IPs you will send from.