Sendmail Authentication

[Jonathan Hutchins]

I need to set up better sendmail authentication for a multi-domain site with a 
few external users.  The problem is, it has to be 100% transparent to the 
users.  They use a variety of clients and have absolutely NO technical 
ability - changing settings in their mail programs is most likely not an 
option.

I presume this means my only option is smtp-after-pop, right?