Reliability of e-mail delivery in the era of spam

E-mail system has been designed once to be one of the most reliable internet services. All possible steps were taken in the protocol to assure either a positive delivery of e-mail or the return of an error message. This is now circumvented in many ways by various anti-spam techniques and the SMTP protocol cannot be considered reliable anymore. The paper discusses some of the current SMTP protocol modifications that are considered harmful and proposes a way for maintaining the reliability of the protocol while increasing the immunity to spam.