Trueguard Knowledgebase

Learn everything about fraud, technologies and more.

SMTP 550 5.1.1 User Unknown: Causes and How to Fix It

550 5.1.1 user unknown means the recipient mailbox does not exist. What the bounce looks like on Gmail, Microsoft 365 and Postfix, and why you must not retry.

SMTP Error Codes: Every Reply Code and What It Means

Every SMTP reply code from 211 to 556, what each one means, and what to do next. Plus enhanced status codes and what Gmail, Yahoo and Outlook return.

What Are Residential Proxies? How They Work & Are Detected

Residential proxies route traffic through real ISP IPs, so blocklists miss them. How the pools are sourced, how they're abused, and what detects them.

What Is JA4 Fingerprinting? Every Field Explained

JA4 fingerprints the TLS ClientHello, JA4T the TCP SYN. What every field in each string means, how they are collected, and what they cannot prove.