What’s the difference between SMTP errors and blocklist responses?

SMTP errors and blocklist responses both cause delivery failures but have different causes and solutions.

General SMTP errors:

• 4xx codes: Temporary failures (try again later)
• 5xx codes: Permanent failures (don't retry)
• Examples: mailbox full, user unknown, server unavailable
• Often address-specific or temporary infrastructure issues

Blocklist-related errors:

• Specific rejection messages citing blocklist lookup
• Example: 550 5.7.1 blocked by Spamhaus SBL
• Affects all email from blocked IP or domain
• Requires delisting, not just retry

Identifying blocklist errors:

• Error message mentions specific blocklist name
• Rejections affect all recipients, not just some
• Sudden widespread failures where delivery worked before

Response strategies:

SMTP errors: Handle bounces, clean list, retry soft bounces

Blocklist errors: Identify which blocklist, request delisting, address root cause

Monitoring:

• Track error messages for blocklist mentions
• Regularly check blocklist status proactively
• Investigate sudden delivery failure patterns