521
SMTP Error 521: Server Does Not Accept Mail
Permanent failure
Critical severity
System
RFC 7504
What it means
SMTP Error 521 means “Server Does Not Accept Mail.” The receiving server does not accept any incoming email. The domain exists but its mail server is explicitly configured to reject all mail, or the domain has published a null MX record indicating it does not receive email.
At a glance
Code521
Bounce typeHard (permanent)
SeverityCritical
CategorySystem
What to doSuppress the address; do not retry
StandardRFC 7504
521 5.1.2 mx.example.com does not accept mailWhat does 521 mean?
SMTP code 521 means the server at the other end does not accept incoming email at all. This is a permanent failure that indicates the domain is not set up to receive email. Some domains are configured for sending-only or exist purely as website domains with no email functionality.
RFC 7504 defines 521 as a permanent rejection response for servers that never accept mail. Domains that want to signal they do not accept email may publish a null MX record (a single MX record with a period as the target). When you encounter a 521, the recipient address should be removed from your list immediately - there is no point in retrying.
How 521 plays out
Your server attempts delivery
The recipient server returns a permanent
521 rejectionThis is a hard bounce: the message will not be accepted as sent
Suppress the address and fix the root cause before resending
Where 521 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 |
| 521 is | ✓ this code |
Common causes of 521
- Domain has a null MX record indicating it does not accept email
- Server is configured for outbound-only email
- Domain is not set up for email reception
- Organization has deprecated email for this domain
How to fix 521
- Remove the address from your mailing list immediately - this domain does not accept email
- Verify the domain MX records using an MX Lookup tool
- Check if you have the correct domain for the recipient
- Look for an alternative contact address on the correct domain
Frequently asked questions
What does SMTP error 521 mean?
SMTP error 521 means "Server does not accept mail," indicating that the recipient's mail server is explicitly configured not to receive incoming email. This permanent error typically occurs when you attempt to send email to a domain whose server acts only as a mail relay (passing messages to another server) or a domain that is set up exclusively for outbound sending. The server is declaring that it is not the final destination for email and will not store messages locally.
Why would a server not accept mail (521 error)?
A server returns 521 because it is intentionally configured to reject inbound email. This happens when the domain uses a send-only mail configuration (common for automated notification systems), the server functions as an intermediary mail relay rather than a final destination, or the server's MX records are misconfigured and point to a host that does not handle incoming mail. Some domains deliberately set up their servers this way because they only need to send transactional emails and do not receive replies.
How do I fix SMTP error 521?
If you are the sender, first verify the recipient's email address is correct and the domain actually accepts inbound email by checking its MX records with a DNS lookup tool. If the MX records point to a valid mail server, the issue may be a server misconfiguration on the recipient's end; contact their postmaster or IT administrator. If you are the server administrator receiving this error about your own domain, ensure your MX records point to a server configured to accept incoming mail and that your mail server software has inbound delivery enabled.
Is a 521 error a hard bounce or soft bounce?
SMTP error 521 is a hard bounce (permanent failure) because it carries a 5xx status code. The receiving server is definitively stating that it does not accept mail, and retrying delivery will produce the same result. However, before removing the address from your list, verify that the emails were not successfully relayed to a final destination server, as some relay configurations return 521 even though the message is ultimately delivered downstream. If the recipient confirms non-delivery, treat it as a permanent bounce.
What does it mean when MX records show a domain does not accept mail?
When a domain's MX records indicate it does not accept mail, it means the domain is configured with either no MX records, a "null MX" record (as defined in RFC 7505), or MX records pointing to a server that returns 521. This is common for domains used exclusively for sending automated emails, hosting websites without email functionality, or subdomains not intended to receive mail. Attempting to send email to such a domain will always result in a bounce, and the address should be removed from your mailing list.
Related bounce codes
Sources & further reading
Reviewed by Jennifer Jackson, Email Deliverability Analyst · June 2026
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