What common mistake do developers make with Mail configuration, why does the mistake cause problems, and what is the safer approach?

What common mistake do developers make with Mail configuration, why does the mistake cause problems, and what is the safer approach? One recurring problem with mail configuration is applying it without understanding the boundary it protects. Hard-coding SMTP passwords or API keys inside a Mailable class instead of using environment-backed configuration. In addition, Environment and application configuration determine the mail transport, sender and service credentials. Safer engineering approach: keep the responsibility in the correct layer, use the framework's supported API, validate and authorize inputs, avoid hidden side effects, and verify the behavior with tests.