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 antispam 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.