Trueguard Knowledgebase

Learn everything about fraud, technologies and more.

Hard Bounce vs Soft Bounce: The Rule and the Codes

Hard bounce means a 5xx permanent failure and suppression. Soft bounce means 4xx and a retry. Neither term is in any RFC, and ESPs do not fully agree.

SMTP 421: Service Not Available, Closing Channel

SMTP 421 is a temporary failure, not a dead address. What triggers it on Gmail, Microsoft 365, Yahoo and Postfix, and how long a sender should keep retrying.

SMTP 550 5.7.1: Message Blocked or Unable to Relay

550 5.7.1 is a policy rejection, not a bad address. What triggers it on Gmail, Microsoft 365 and Postfix, and why SPF failures carry a different code.

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.