Section 28. Whoever purposefully disseminates to a person he knows or believes to be a minor any matter harmful to minors, as defined in section 31, knowing it to be harmful to minors, or has in his possession any such matter with the intent to disseminate the same to a person he knows or believes to be a minor, shall be punished by imprisonment in the state prison for not more than 5 years or in a jail or house of correction for not more than 21/2 years, or by a fine of not less than $1000 nor more than $10,000 for the first offense, not less than $5000 nor more than $20,000 for the second offense, or not less than $10,000 nor more than $30,000 for a third or subsequent offenses, or by both such fine and imprisonment. A person who disseminates an electronic communication or possesses an electronic communication with the intent to disseminate it shall not be found to have violated this section unless he specifically intends to direct the communication to a person he knows or believes to be a minor. A prosecution commenced under this section shall not be continued without a finding or placed on file. It shall be a defense in a prosecution under this section that the defendant was in a parental or guardianship relationship with the minor. It shall also be a defense in a prosecution under this section if the evidence proves that the defendant was a bona fide school, museum or library, or was acting in the course of his employment as an employee of such organization or of a retail outlet affiliated with and serving the educational purpose of such organization.