AS1 (“Applicability Statement 1”) is an SMIME-based transfer protocol that uses plain old email protocols (such as SMTP and POP3) to transmit files with end-to-end encryption and guaranteed delivery/non-repudiation (when MDNs are in use).
End-to-end encryption is accomplished through the use of asymmetric encryption keyed with the public and private parts of properly exchanged X.509 certificates. Guaranteed delivery is accomplished through the use of strong authentication and signing, also through the use of the public and private parts of properly exchanged X.509 certificates.
AS1 is an unpopular implementation of the AS2 protocol, at least for new implementations. Many vendors successfully sell software that supports AS2 but not AS1 or AS3. However, AS1’s design as an email-based protocol allows many companies to implement it without investing in extra file transfer technology at their perimeters; they simply need to implement AS1 internally and make sure it can access email.