When managing a mail server or sending outbound emails, one of the most frustrating bouncebacks you can receive is the 550 5.7.1 Service Unavailable error. This error essentially means that the recipient’s mail server refused to accept your email due to security policies, blacklists, or misconfigured DNS records.

Before diving into complex server configurations, the very first step is to check your domain’s email health. You can use our real-time Mail Tester to instantly diagnose if your SPF, DKIM, or DMARC records are the culprit.

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Common Causes for 550 5.7.1

1. Missing or Invalid SPF Records

Sender Policy Framework (SPF) is a DNS record that tells the world which IP addresses are allowed to send emails on behalf of your domain. If a spammer tries to spoof your email, the receiving server checks your SPF record.

If the IP doesn’t match—or if you don’t have an SPF record at all—the server will throw a 550 error and drop the connection.

2. IP Blacklisting

If your server’s IP address has been flagged for sending spam in the past, it might be on a global DNSBL (DNS Blackhole List) like Spamhaus or Barracuda.

You can check if your IP is blacklisted right now:

3. Strict DMARC Policies

If your DMARC policy is set to p=reject and your emails are failing SPF or DKIM alignment, the receiving server is legally obligated to reject the email and return a 550 error. Ensure your outgoing emails are properly signed with a DKIM key.

Next Steps

By running the diagnostics above, you should be able to pinpoint exactly why the recipient server is blocking you. Once you fix the underlying DNS or IP reputation issue, the 550 error will disappear!