PSTmail is a system which allows the exchange of e-mail between two TCP/IP networks with different classification levels or security policies which would impede their connection through any other means. The two networks are not equivalent: one is considered to have a higher classification degree or security level.

PSTmail guarantees the impossibility of any type of traffic between the two networks except the mail transferred by the system itself.

PSTmail has been developed to work in MAXIMUM security environments where networks of different security levels or classification must be kept separated. It is appropriate for fulfilling the most stringent security requirements, for example, when the protocol stack must be discontinued at all OSI model levels and where information flow between two networks must be controlled.

PSTmail supports Internet protocols: POP3 and IMAP4 for receiving and SMTP for sending. The delivery of incoming mail is transparent to the users. Outgoing email requires authorization by digital signature (S/MIME).