550 5.7.1 and similar rejections

Problem

Messages are rejected with a 550 5.7.1 or a similar policy error.

What it means

The receiving server accepted the connection and refused the message on policy grounds — not because the address is wrong.

Possible causes

• The sending mailbox has a setup or policy issue
• The sending mailbox or its infrastructure is on a blocklist
• The recipient's organisation blocks external cold mail
• Volume from your domain tripped the receiver's rate policy
• The content matched a filtering rule

How to check

1. Read the exact text of the rejection on the failed message; receivers usually say why.
2. Open Email Accounts and check the sending mailbox for setup or policy warnings.
3. Look at whether the rejections are concentrated at one recipient domain or spread across many.

How to fix

• Resolve the mailbox action shown before sending again.
• If it is one recipient domain only, that organisation is blocking you; remove those leads.
• If it is widespread, reduce volume per mailbox, pause the campaign, and let warmup and reputation recover before resuming.

When to contact support

Authentication passes, rejections are widespread, and volume is already low.