Can We use smtp server setting for more than one customer, for example can we buy this service from any company ( yahoo. godaddy etc..) can we use this email server setting to all customer machines so they can use the scan to email feature? Anybody has an idea about that?

That shouldn't be a problem as long as the ISP the client is on allows port 25 traffic or the mail service provider has an alternate non-ssl smtp port (AT&T for example likes to block port 25)

I setup my own email server and customized it to use port 80 (which is normally used for regular web traffic - http)

Originally I set it up for testing - if the IT person gave us mail settings and they didn't work I could simply point it at my own mail server to prove it wasn't the machine but the settings the IT person gave me.

As time went on we have been forced to use it for a bunch of clients as various ISP's crack down on normal SMTP port 25 traffic.

Using port 80 has insured availability...