5.7.27
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
550 5.7.27 <[email protected]>: 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 rejectionThis 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
Reviewed by Jennifer Jackson, Email Deliverability Analyst · June 2026
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