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