What causes block bounces? (e.g., blocklisting, reputation, policy)

Block bounces result from various sender-side issues:

Blocklist inclusion:

IP on Spamhaus, Barracuda, or other lists. Domain blocklisted for past abuse. URL in message on known-bad lists.

Reputation problems:

High complaint rates. Previous spam-like behavior. Low engagement metrics. Sending to invalid addresses.

Content triggers:

Spam trigger words and phrases. Suspicious URL patterns. Malware signatures. Excessive images or formatting.

Authentication failures:

SPF, DKIM, or DMARC check failed. Alignment issues with From domain. Missing required authentication.

Policy violations:

Rate limits exceeded. TLS requirements not met. Recipient opt-out not honored.

Block bounces reflect sender behavior. The solution is improving your sending practices, not changing addresses.