What "System Type" for the "Sender" should I use if Sender send EDI messages via smtp?

Hi, I have a scenario like this. An external company(Sender) sends XML files as attachment in the email via SMTP to our mail address, then we pickup the xml file and finally send it to B1 after mapping in the style-sheet (Receiver). I know there is an receive mail atom that supports to receive email. my question is simple, What "System Type" for the "Sender" should I use in this case?