What 550 5.7.1 Means
Gmail's reverse-DNS check failed. When your mail server connects to Gmail's MX, Gmail does a PTR lookup on the connecting IP to find its hostname. Per RFC 5321 best practices, the hostname returned by PTR must also forward-resolve back to the same IP (matching forward-and-reverse), and the hostname declared in HELO should match. If any step fails, Gmail returns 550 5.7.1 'IP has no PTR record' or similar.
Gmail and Google Workspace are strictest on PTR — they reject unconditionally. Microsoft 365 is moderately strict. Yahoo and most enterprise gateways check PTR but often soft-fail rather than reject.
Sending IP has no PTR record at all (most common with self-hosted setups or budget VPS providers); PTR exists but doesn't forward-resolve back to the same IP (broken forward-reverse pair); PTR resolves to a hostname different from what the server declares in HELO; or your hosting provider hasn't set up reverse DNS for the IP range.
How to Fix 550 5.7.1
- 1
Look up your sending IP's PTR record
Run 'dig -x YOUR.SEND.IP.HERE' or use any online PTR lookup tool. If the result is empty (NXDOMAIN), your IP has no reverse DNS — that's your problem. If it returns a hostname, continue to step 2.
- 2
Verify forward-and-reverse match
If PTR returns 'mail.yourdomain.com', run 'dig mail.yourdomain.com' and confirm the A record returns the same IP. If they don't match, the pair is broken — receivers treat broken pairs as missing PTR.
- 3
Verify HELO matches PTR
Your mail server announces a hostname in the HELO/EHLO command. Receivers compare that against PTR. If your server says 'EHLO smtp.example.com' but PTR returns 'mail.yourdomain.com', the mismatch is treated as suspicious. Configure your server's HELO to match its PTR.
- 4
Set up PTR through your hosting provider
PTR records are controlled by the IP's owner (your hosting provider), not by you. Open a support ticket asking them to set the PTR for your IP to your desired hostname. Most cloud providers (Azure, AWS, GCP) let you set this in their console without a ticket. Budget VPS providers may not offer PTR control at all — in that case the IP isn't suitable for outbound mail.
- 5
If you're on ColdRelay, this should be automatic
ColdRelay configures forward-and-reverse DNS for every dedicated IP at provisioning. The Servers page in the dashboard shows the IP, its PTR, and the HELO hostname — all three should match. If you see drift on a ColdRelay-managed IP, contact support; we'll re-publish.
Note: Some receivers cache PTR aggressively. After fixing reverse DNS, expect 6-24 hours before the rejection clears at all receivers.
References
550 5.7.1 in the Cold Email Context
Reverse DNS misconfiguration is one of the cleanest infrastructure signals separating professional sending platforms from budget VPS setups. Major receivers refuse mail from any IP without a PTR record because spammers traditionally rent IPs from providers that don't bother with reverse DNS. If your cold email setup is on a $5/month VPS without PTR control, you're DOA at Gmail regardless of your authentication. ColdRelay provisions every dedicated IP with matching forward-and-reverse DNS on isolated Azure tenants — the PTR is set at IP allocation, before any mail flows, and the HELO hostname matches by default. The Servers page in the dashboard shows the full DNS chain (PTR, A, HELO) so any drift is immediately visible.
Frequently Asked Questions
Why do receivers even check reverse DNS?
Reverse DNS is a baseline cost-to-spam signal. Real mail-server operators set PTR records; many spammers rent IPs from providers that don't offer PTR control. Receivers use PTR presence as a cheap first-pass filter against the lowest tier of spam.
Can I set PTR myself?
Not directly — PTR is set by the IP's owner (your hosting provider or upstream ISP). You request it from them; they publish it. Major cloud providers (Azure, AWS, GCP) provide self-service PTR controls in their consoles. Smaller VPS providers may require a support ticket.
What hostname should PTR resolve to?
A hostname under a domain you control, ideally matching the HELO your mail server declares. Common pattern: 'mail.yourdomain.com' or 'mta1.yourdomain.com'. The hostname's A record must forward-resolve back to the same IP for the pair to be considered matching.
How long does PTR propagation take?
5-60 minutes for most major receivers, up to 24 hours for stragglers. Some receivers cache negative PTR results (NXDOMAIN) more aggressively than positive ones, so if you had no PTR for a while before publishing, the cached fail may persist longer.