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AS3 (“Applicability Standard 3”) is an SMIME-based transfer protocol that uses FTP/S to transmit files with end-to-end encryption and guaranteed delivery/non-repudiation (when MDNs are in use).

AS3 is an unpopular implementation of the AS2 protocol.  Many vendors successfully sell software that supports AS2 but not AS1 or AS3.  However, AS3’s design as an FTP-based protocol allows many companies to implement it with minimal file transfer technology investments at their perimeters; they simply need to implement AS3 internally and make sure it can access a plain old FTP/S server exposed to the Internet.

See also “AS1” for the email-based variant and “AS2” for the HTTP/S-based variant.