AS1 EDIINT Custom Processing Service and UserId

I have a custom processing service defined for my EDIINT (AS1/AS2) communications. As per wM documentation this service should have two input parameters a string “content-type” and the stream representing the actual “content”.
I need to know the actual client id calling/sending the data. For As2 (HTTPS) I can use the server API getUser() method as our HTTPS authentication/login is certificates based.
But for AS1 you can only specify “one userId” to run the service as on the email/pop3 port and this is the userid that is being returned by the getUser() wm server api method. I’d like to use one email port for different customers using AS1. How can we obtain the TN identified customer name/id within our custom EDIINT service for AS1 communications.