550 5.7.25: reverse DNS check failed

550 5.7.25 is a permanent SMTP reply: no reverse DNS for the IP. This page is part of our SMTP error code reference, where every code links to the RFC and to what Gmail, Outlook and Yahoo publish about it.

Kind

Permanent: sending the same message again fails the same way

Retry by hand

No, fix the cause first

In Mailbay

bounced, address not suppressed

What 550 5.7.25 means#

RFC 7372 registers X.7.25 as "Reverse DNS validation failed", with the basic code 550 ^[1].

A PTR record maps an IP address to a host name. Receivers usually also check that the name resolves back to the same IP, which is called forward confirmed reverse DNS.

What Gmail, Outlook and Yahoo say#

Gmail

"550 5.7.25 This message was blocked because the sending IP address doesn't have a PTR record, or the forwarding DNS entry doesn't reference the sending IP address." ^[2]. Its guidelines require "valid forward and reverse DNS records, also referred to as PTR records" from all senders ^[3].

Yahoo

Requires all senders to "Have a valid forward and reverse DNS record for your sending IPs" ^[4].

Outlook and Microsoft 365 does not list 550 5.7.25 on the error pages we read, so we do not guess what it means by it.

Common causes#

• A new server or VPS whose provider sets no PTR by default.
• A PTR that names a host whose A or AAAA record points somewhere else.
• Sending over IPv6 from an address that has no PTR at all.

How to fix it#

1. Look up the PTR with dig -x <ip>, then look up the name it returns and check that it gives back the same IP.
2. Set the PTR in your hosting provider's panel; only the owner of the IP range can.
3. Or send through a provider whose servers already have it, and stop maintaining your own sending IP.

How Mailbay reports it#

When a recipient's server answers 550 5.7.25 to mail you send through Mailbay, the recipient becomes bounced and you get a message.bounced event with the server's reply in response and permanent: false. The message statuses page lists every state.

A policy refusal says nothing about whether the address exists, so Mailbay does not suppress it, and it does not count toward the automatic pause, which only counts hard bounces and spam complaints.

When you send through Mailbay, mail leaves from Mailbay's own sending servers, so their reverse DNS is ours to keep right, not yours. Your part is the domain records (SPF, DKIM and DMARC).

Related codes#

• 550 5.7.26: failed SPF, DKIM or DMARC, permanent.
• 421 4.7.0: temporary refusal, connection closed, temporary.
• 550 5.7.515: sender rules at Outlook.com, permanent.
• 550 5.7.1: refused for a policy reason, permanent.
• 451 4.7.1: deferred: rate limit or policy, temporary.

Seeing an API error rather than an SMTP code? Mailbay's own errors, each with a fix, are on the errors page.