SMTP Injection is a broad class of vulnerability that can occur in many situations. There is no universal solution to it, so defense-in-depth is important in situations where it might occur.
The good news is you now have the core knowledge required to prevent SMTP injection! You are aware of its existence, when it can occur, and the broad strategies used to counteract it. You also know what environment-specific information you will need to find when implementing those strategies.
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