Feature/implement token generation and validation for knock emails

[devZenta]

This pull request improves the security and flow of the SMTP server's token-based email sending process. The main updates are focused on generating secure tokens, storing them properly, and enforcing validation before allowing email sending.

Token management and validation improvements:

• Securely generates a random token using randomBytes, stores it in tokenStore with the sender's address and a validated: false flag, and constructs a validation link for the user (src/services/smtpServer.ts).
• Adds strict checks to ensure that only validated tokens can be used to send emails: rejects requests with missing or unvalidated tokens, logs warnings, and returns SMTP error code 553 for these cases (src/services/smtpServer.ts).

[Copilot]

Pull request overview

This pull request implements a token-based authentication system for the SMTP server to control email sending. The implementation generates secure random tokens when users "knock" (request permission), stores them with sender information, and enforces validation checks before allowing emails to be sent.

Key Changes:

• Implements secure token generation using 32-byte random tokens converted to hexadecimal format
• Adds validation checks to reject emails with missing, invalid, or unvalidated tokens
• Returns proper SMTP error code 553 with descriptive messages for validation failures

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