How do ESPs differ in bounce classification?

Bounce classification isn't standardized across ESPs. The same SMTP error might be categorized differently by different platforms.

Common variations:

Hard vs soft threshold: Some ESPs classify certain errors as hard bounces; others call them soft. "Mailbox full" is soft to some, hard after N occurrences to others.

Suppression rules: Different policies for when to suppress. Some suppress after one hard bounce; others after multiple soft bounces over time.

Category granularity: Some ESPs have many bounce categories (unknown user, domain not found, policy block, etc.). Others simplify to just hard/soft.

Block vs bounce: Reputation-based rejections may be classified as bounces or as a separate "blocked" category.

Why this matters during migration:

An address suppressed as "hard bounce" at old ESP might not match new ESP's criteria

• Import all suppressions regardless of category to be safe
• Don't assume categories translate directly
• Review new ESP's documentation on classification logic

Post-migration considerations:

• Historical bounce rates may not compare directly between ESPs
• Suppression list size may differ due to different policies
• Set baseline metrics fresh at new ESP rather than comparing to old
• Understand the new ESP's logic to interpret ongoing metrics correctly.