How do spam-block errors appear in bounce logs?

Spam block errors show up in your bounce log as 5xx rejection codes, just like other hard bounces. But the message text is what distinguishes them. Instead of "user unknown" or "no such mailbox," you'll see language about spam detection, blocklists, policy violations, or reputation problems.

Here's what they look like in practice. Your ESP's bounce report might show something like:

• 550 5.7.1 Message rejected due to spam content
• 554 5.7.0 Your message is suspected spam
• 550 IP address [your IP] is blocked on the Spamhaus XBL
• 550 5.7.1 Service unavailable; Client host blocked using SORBS
• 550 Rejected for spam activity

The key patterns to look for:

• References to a specific blocklist (Spamhaus, SORBS, Barracuda, etc.). This tells you which list to check for delisting.
• "Spam" or "suspected spam" in the message text. This is content or reputation-based filtering rather than a blocklist entry.
• References to your IP address or sending domain. IP-based blocks and domain-based blocks have different resolution paths.
• Policy language ("rejected per policy," "blocked by policy") without a specific blocklist name. Usually means the receiving server's local rules, harder to diagnose without reaching out to postmaster.

Not every ESP surfaces the full SMTP error message text in its interface. Some simplify these into categories like "spam block" or "policy rejection" without the underlying code. If you're troubleshooting a pattern and can't see the raw messages, check whether your ESP offers a full bounce log export or contact their support for the SMTP detail.

If you're seeing repeated spam-block errors to the same domain or provider, the first step is checking our free blocklist checker to see whether your sending domain or IP is listed. If you're not blocklisted, the issue is more likely content or sender reputation, which takes more investigation.