Bounce "550 5.7.1": message rejected or blocked by policy

What it means

When an email cannot be delivered, your provider sends you back an error message, called a bounce or "non-delivery report". It contains a code and a short text from the receiving server. For example:

550 5.7.1 Service unavailable, Client host [203.0.113.10] blocked using zen.spamhaus.org

The code 5.7.1 means "refused for security or policy reasons". It is generic: the real reason is in the text after the code.

How to read a bounce

• The first digit: 4 means temporary (the server will try again), 5 means permanent (the email is lost).
• The middle part: 5.1.x is an address problem (the mailbox does not exist), 5.2.x a mailbox problem (full), 5.7.x a security or policy refusal.
• The text: the most useful part. Look for the name of a blacklist, an IP address, a link to a help page, or words like SPF, DKIM, DMARC, policy, reputation.
• The server name: shows who refused the email (Gmail, Outlook, a company's filter).

Common reasons behind a 5.7.1

• Blacklisted IP: the text names a list such as Spamhaus, Barracuda or SpamCop.
• Authentication: SPF fails, or DMARC refuses the email.
• Reputation: "poor reputation", "blocked for spam", "banned sending IP".
• Company rules: the recipient's company only accepts certain senders, blocks attachments or blocks your country.
• Relaying denied: your own server refuses to send because you are not logged in. Check your SMTP username and password.

How to check

• Send a test email: on the home page, copy your unique test address, send it a normal email from the mailbox or tool you want to check, then open the report. It shows authentication, blacklists, reverse DNS and spam score.
• Look up the IP named in the bounce on /blacklists.

How to fix it

1. Copy the full bounce text and find the reason in it.
2. If a blacklist is named, follow the blacklist guides: fix the cause, then request removal. Blacklist removal help is available in the paid Delivmail dashboard.
3. If authentication is mentioned, fix SPF, DKIM and DMARC for the tool that sent the email.
4. If reputation is mentioned, slow down, clean your list and contact the receiver's sender support page if the bounce gives a link.
5. If it looks like a company rule, contact the recipient another way and ask their IT team to allow your domain.

Check that it is fixed

Send a new test email from the home page: the report should be clean. Then resend to the recipient who bounced. If the same bounce comes back, read the text again: the reason may have changed.