Post SMTP < 4.0.2 - Unauthenticated Stored XSS via 'user_email' Parameter

Description

The plugin is vulnerable to Stored Cross-Site Scripting via the 'user_email' parameter due to insufficient input sanitization and output escaping. This makes it possible for unauthenticated attackers to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page. This is exploitable without authentication on WordPress Multisite installations with public registration enabled, as WordPress accepts email addresses containing numeric HTML character references that Post SMTP's stricter validator rejects, persisting the attacker-controlled address verbatim to the email log via the failed-send exception message.