SMTP Preprocessor is advanced tool for mail administrators. It enhances functionality of corporate mail server. The program receives SMTP messages before corporate SMTP server, on-the-fly changes or rejects them, and retransmits result to the server. Wide range of settings allows to improve security and add new features to corporate SMTP server. SMTP Preprocessor can reject connection based on remote host IP address (Black List), presence of reverse DNS PTR record, validity of HELO or EHLO command and connections limit; reject MAIL command based on sender address (Black List) and transactions limit; reject RCPT command based on recipient address (Private Recipients List), recipient domain name (Anti-Relaying), sender address (Restricted Senders List) and recipients limit; reject out-of-order SMTP command; flexibly replace corporate SMTP server response or any SMTP Preprocessor generated response; replace sender in SMTP envelope (MAIL command); replace recipient in SMTP envelope (RCPT command). The program keeps adjustable detailed log of all SMTP events.