SMTP Service?

If im sending emails out using the SMTP service do each one of my clients need the SMTP service installed? We have an SMTP server already and it is the one that I'm using to relay the email messages but still would like to know if each client that uses my application has to have the SMTP service installed (that would be kinda anoying)no they dont need it. Since the pages they are accessing are on a web server with smtp service it takes care of all that.

HTH
Thanks, I was kinda confused but now it makes sence.