421 4.7.1 Service unavailable: no reverse DNS for [x.x.x.x]What does 421 No reverse DNS mean?
The receiving server performed a reverse DNS (PTR) lookup on your sending IP and found no result. Rather than rejecting the message permanently, the server returns a 421 soft bounce, which tells your MTA to retry later in case the lookup was a transient DNS issue. If the PTR record is genuinely missing, every retry will return the same 421 until the record is configured. Many MTAs treat absence of PTR as a strong spam signal because legitimate mail servers always have reverse DNS configured to match a forward A record (forward-confirmed reverse DNS, or FCrDNS).
Is this a soft or hard bounce?
Your MTA will keep retrying for up to 5 days. Configure the PTR record now to fix the entire backlog at once. See also 550 client rejected: no rDNS for the hard-bounce variant servers use after exhausting patience.
Common causes
Default state for many cloud VMs, VPS instances, and home connections. The hosting provider must configure PTR on the IP, not the customer (PTR is set on the IP owner side, not via your DNS).
FCrDNS requires the PTR hostname to also have an A record pointing back to the same IP. Mismatched forward and reverse records fail the check.
Residential and dynamic IPs typically have generic PTR records or none at all. Use an authenticated SMTP relay instead of sending direct.
How to fix it
Check your MTA configuration or use dig +short myip.opendns.com @resolver1.opendns.com. Confirm which IP is actually used for outbound port 25.
Run dig -x <your-ip>. If the result is empty, no PTR exists. If it returns a hostname, run dig <hostname> to confirm forward resolution back to the same IP.
PTR records are set by the IP owner, not by your domain registrar. For AWS use the request form in EC2. For DigitalOcean, set the droplet name to the FQDN. For dedicated hosts contact your hosting provider.
After PTR propagates (usually within minutes, occasionally up to 24h), the MTA will retry queued messages automatically. See 450 4.7.1 client host rejected for the related Postfix variant.
Provider-specific notes
| MTA | Behavior |
|---|---|
| Postfix | Returns 421 4.7.1 when reject_unknown_reverse_client_hostname is in smtpd_client_restrictions. After retries exhaust, escalates to 550 client rejected: no rDNS. |
| Exim | Returns 421 with the no-rDNS check in helo_try_verify_hosts. Soft fails by default. |
| Sendmail | Returns 421 if the configuration sets a temporary failure for PTR lookups. Switching to permanent (550) is common after sustained issues. |
Related error codes
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About the Author
Alaa · LinkedIn
Email infrastructure specialist with 8+ years of hands-on experience in SMTP, deliverability, and email verification. I’ve configured and troubleshot mail systems across Postfix, Exchange, and cloud relays, managed IP reputation and warmup campaigns, and built verification pipelines processing millions of addresses. My work spans DNS authentication (SPF, DKIM, DMARC, BIMI), bounce handling, blocklist monitoring, and compliance frameworks including CAN-SPAM and GDPR. I write every article on SMTPedia to give email professionals, developers, and marketers the accurate, RFC-grounded reference they need.
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