Enhanced Status Code 5.7.27: Sender Address Has Null MX

Permanent failure High severity Address RFC 7505

What it means

Enhanced Status Code 5.7.27 means “Sender Address Has Null MX.” Your sending domain has published a null MX record, declaring it does not accept email. Since it cannot receive bounces or replies, many receiving servers reject email from domains with null MX. Fix your DNS records immediately.

At a glance

Code5.7.27

Bounce typeHard (permanent)

SeverityHigh

CategoryAddress

What to doSuppress the address; do not retry

StandardRFC 7505

What it looks like in your mail logs

550 5.7.27 <sender@example.com>: Sender address rejected: Domain example.com does not accept mail (nullMX)

What does 5.7.27 mean?

Enhanced status code 5.7.27 means the receiving server checked your sender domain and found it has a null MX record (RFC 7505). A null MX record explicitly declares that the domain does not accept email. Many receiving servers reject email from such domains because bounce notifications cannot be delivered back to the sender.

This is typically a DNS misconfiguration. If you are sending email from a domain, it should have proper MX records configured to receive bounces and replies. Check your domain's DNS records immediately.

How 5.7.27 plays out

Your server attempts delivery

The recipient server returns a permanent 5.7.27 rejection

This is a hard bounce: the message will not be accepted as sent

Suppress the address and fix the root cause before resending

Where 5.7.27 sits: soft vs hard bounce

	Soft bounce (4xx)	Hard bounce (5xx)
Nature	Temporary	Permanent
SMTP class	`4xx`	`5xx`
What to do	Let it retry	Suppress the address
Recoverable?	Often	No
5.7.27 is		✓ this code

Common causes of 5.7.27

Sender domain has a null MX record (priority 0, target \".\")
DNS misconfiguration when setting up the domain
Domain was set up for web-only use but is now being used for email

How to fix 5.7.27

Add proper MX records to your sending domain
Remove any null MX record from your DNS configuration
Ensure your sending domain can receive bounce messages
Verify DNS changes using MX Lookup after making corrections

Frequently asked questions

What does error 550 5.7.27 "Sender address has null MX" mean?

Error 550 5.7.27 means the receiving mail server rejected your email because your sending domain has a "null MX" record in DNS, a special MX record (priority 0 with a single dot ".") that explicitly declares the domain does not accept email. As defined in RFC 7505, a null MX tells other servers this domain neither sends nor receives mail. Receiving servers reject messages from null-MX domains because they cannot send bounce notifications back to the sender.

What is a null MX record?

A null MX record is a DNS MX record with priority 0 and a target of "." (a single dot), formally specified in RFC 7505. It is an explicit declaration that a domain does not accept any email. This is intended for domains that have no mail infrastructure: such as parked domains, redirect-only domains, or service domains. Without a null MX, servers may fall back to the domain's A/AAAA record for mail delivery, causing unwanted delivery attempts and confusing bounces.

How do I fix a 5.7.27 null MX error?

If your domain should be sending email, remove the null MX record from your DNS configuration and add a proper MX record pointing to your mail server. If you are using a domain that should not send email but you need to send from it, the null MX record was likely set intentionally and needs to be replaced with a functioning MX record and supporting email infrastructure (SPF, DKIM, DMARC). Contact your DNS administrator or domain registrar to make the change.

Should I set up a null MX record for my domain?

Set up a null MX record only if your domain is definitively not used for email: for example, domains used solely for website hosting, redirects, or API services. A null MX record prevents abuse of your domain for email spoofing and reduces unnecessary SMTP connection attempts. However, do not set a null MX on any domain that sends or receives email, including transactional email. Instead, use a restrictive DMARC policy (p=reject) with proper SPF and DKIM to protect domains that do send email.

Related bounce codes

Sources & further reading

RFC 7505 · The standard that defines this reply code

Reviewed by Jennifer Jackson, Email Deliverability Analyst · June 2026 ← All bounce codes