If your outgoing email is rejected with a 550 571 ESA sender blocked error, your email has been blocked by an Email Security Appliance (ESA) or an anti-spam filtering system.
At UpTime, we keep server reputations pristine and protect outbound deliverability. This error indicates that an automated security filter flagged your domain name as a potential spam risk.
Common Causes & Direct Solutions
1. Your Domain Is Listed on a Real-Time Blocklist (RBL)
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The Cause: If a computer on your network was infected with malware or was sending high volumes of unauthenticated mail, major blacklists (such as Spamhaus or Barracuda) may temporarily flag your domain.
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The Fix: Check your sending IP or domain against global RBL monitoring tools. If you are listed, request a delisting on the blocklist provider's portal or clear malware from all local devices sharing your internet connection.
2. Your Website Is Sending Unauthenticated PHP Mail
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The Cause: Using default, unauthenticated PHP mail functions on your website often triggers strict security filters, resulting in rejection.
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The Fix: Ensure all web contact forms, e-commerce stores, and notifications are sent via Authenticated SMTP using valid mailbox credentials rather than raw PHP scripts.
3. Missing or Misconfigured DNS Email Records (SPF, DKIM, & DMARC)
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The Cause: Recipient servers verify incoming mail against your domain's DNS records. Missing authentication signals to security systems that your email might be spoofed.
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The Fix: Verify that your domain has valid SPF, DKIM, and DMARC records configured in your DNS management portal to prove your sender authenticity.
4. Triggering Spam Filters via Content or Recipient Volume
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The Cause: Rapidly blasting marketing emails to large recipient lists, or including flagged links, excessive capitalisation, and spam-trigger words, can trigger ESA systems.
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The Fix: Avoid sending bulk marketing campaigns through standard transactional email accounts. Use a dedicated email marketing service for high-volume broadcasts.
Prevent Future Blocks: Proactive Security Tips
To keep your outbound mail flowing smoothly and prevent security filters from catching your messages in the future, follow these proactive steps:
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Reset Your Password: If you suspect your account was compromised or used to send outbound spam, use the self-service reset options in your UpTime Client Portal immediately to secure your credentials.
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Update Saved Credentials: Always update saved login credentials across all your devices, mail applications (such as Outlook or Apple Mail), and saved settings after changing a password. Stale credentials on background devices can trigger automated security locks.
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Confirm Mailbox Settings: Ensure your email application uses the exact SMTP server details, the correct port numbers (e.g., Port 465 or 587), and the full email address as the username.
