Help ensure your emails
hit the inbox
Stop guessing why your messages end up in spam. Follow this 26-point checklist to achieve technical compliance, enhanced sender reputation, and delivery success.
Authentication
rDNS / PTR / HELO
TL;DR
- Mailbox providers expect forward DNS, reverse DNS, and HELO identity to align.
- Keep PTR, forward DNS, and SMTP HELO / EHLO fully consistent.
The sending IP address must have correctly configured forward and reverse DNS, with a valid fully qualified domain name (FQDN). The hostname presented in the SMTP HELO / EHLO command must match the reverse DNS (PTR) record and resolve back to the originating IP address (forward-confirmed reverse DNS).
Missing, generic, or mismatched PTR records, incorrect HELO hostnames, or broken forward DNS resolution may trigger SMTP rejections, greylisting, or increased spam filtering.
Example of correct alignment:
In this configuration, reverse DNS, forward DNS, and HELO identity all match. That consistency is what many receiving systems expect before assigning trust.
Ensure that each sending IP has a unique, static PTR record pointing to a stable hostname, and that this hostname is consistently used in SMTP transactions. Proper rDNS, PTR, and HELO alignment is a fundamental requirement for establishing sender identity and maintaining optimal deliverability.
Missing, generic, or mismatched PTR records, incorrect HELO hostnames, or broken forward DNS resolution may trigger SMTP rejections, greylisting, or increased spam filtering.
Example of correct alignment:
- Sending IP: 203.0.113.25
- PTR (reverse DNS): 25.113.0.203.in-addr.arpa -> mail.example.com
- A record (forward DNS): mail.example.com -> 203.0.113.25
- SMTP HELO / EHLO hostname: mail.example.com
In this configuration, reverse DNS, forward DNS, and HELO identity all match. That consistency is what many receiving systems expect before assigning trust.
Ensure that each sending IP has a unique, static PTR record pointing to a stable hostname, and that this hostname is consistently used in SMTP transactions. Proper rDNS, PTR, and HELO alignment is a fundamental requirement for establishing sender identity and maintaining optimal deliverability.
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