What are inbound vs outbound filters in ESPs?

ESPs operate filters in both directions: outbound filters protect the internet from bad email leaving the ESP; inbound filters protect the ESP from external threats.

Outbound filters (what most senders encounter):

• Content scanning for spam patterns, phishing, malware
• Rate limiting to prevent abuse
• Reputation monitoring to catch degrading sender behavior
• Policy enforcement (blocked attachment types, prohibited content)

These protect the ESP's infrastructure reputation and prevent customers from sending harmful content.

Inbound filters (protecting ESP systems):

• Protection for reply handling and feedback loop addresses
• Security for administrative interfaces and APIs
• Defense against attacks targeting ESP infrastructure
• Filtering of malicious incoming webhooks or integrations

Why outbound filtering matters for senders:

• Your email passes through the ESP's outbound filters before delivery
• Triggering these filters can delay, quarantine, or block your messages
• Repeated violations can result in account actions

The filters protect you too: they catch compromised accounts and accidental spam sends before they damage your reputation

Understanding that your ESP actively filters helps explain why certain content or patterns cause problems even before reaching recipient mail servers.