550 5.7.27: SPF failed for a bulk sender (Gmail), or the sender has a null MX

At a glance

• Type: permanent failure (class 5, RFC 3463 §2). The sending server does not retry; sending the same message again fails the same way until the cause is fixed.
• Basic code 550: “Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)” (RFC 5321 §4.2.3).
• Enhanced code 5.7.27: “Sender address has null MX” in the IANA registry (RFC 7505), in the group security or policy status (RFC 3463 §3.8).
• Who uses it: Gmail uses it for SPF failures of bulk senders; the IANA registry name is the null MX meaning of RFC 7505.

If Gmail sent this bounce, 5.7.27 means you send in bulk and your message failed SPF. Elsewhere it can carry its registered meaning: your sender address belongs to a domain that says it accepts no mail.

Two meanings for one code

Gmail's use differs from the official one, so check who sent the bounce.

• The registry meaning. The IANA registry lists X.7.27 as "sender address has null MX", from RFC 7505 §6. A null MX is an MX record of 0 ., which tells the world the domain receives no mail at all (RFC 7505 §3). RFC 7505 §4.1 says a server that refuses mail because the sender's domain has a null MX should answer 550 5.7.27, and §4.2 warns that a domain used in sender addresses should not publish one.
• Gmail's meaning. Gmail uses 550 5.7.27 when a bulk sender's message fails SPF, and 421 4.7.27 when it rate limits for the same reason (Google's SMTP error reference).

Read the text after the code: it should say whether SPF or the sender domain's MX record is the problem.

Common causes

For Gmail:

• A sending service (marketing platform, CRM, ticket system) is missing from your SPF record.
• Your envelope sender uses a domain whose SPF record does not cover the sending IP.
• The record has an error, so SPF cannot pass.

For a null MX:

• You send from a domain set up for web use only, with a null MX.
• A sending app uses a placeholder sender domain that accepts no mail.

How to fix it

Gmail SPF failure:

1. Check the record with the SPF checker.
2. Add the IP or include of every service that sends for you. The SPF generator builds the record.
3. Google's sender guidelines ask bulk senders (around 5,000 or more messages a day to personal Gmail accounts) for SPF, DKIM and DMARC together, so check DKIM with the DKIM checker and see 5.7.40 for DMARC.

Null MX:

1. Send from a domain that can receive mail, so bounces and replies have somewhere to go.
2. If the domain should receive mail after all, replace the null MX with real MX records.

What to do with the address

Keep the recipient's address. In both meanings the problem sits with your sending domain. A recipient domain with a null MX is a different code: 556 5.1.10.

Related codes

• 550 5.7.23: SPF validation failed
• 550 5.7.30: DKIM failed for a bulk sender (Gmail)
• 550 5.7.40: No DMARC policy for a bulk sender (Gmail)
• 556 5.1.10: The recipient domain publishes a null MX: it takes no mail

See every bounce code in the dictionary.

Many bounces from addresses that do not exist can be avoided: check an address with our free email checker, or clean a whole list before you send.

How we wrote this: drafted by the email-validation.org team with the help of AI writing tools, from our product documentation and the sources linked in the text.