What makes (for example) Eudora or Outlook Express, any more of an email client than Incredimail ... ?
Nothing. Those examples are desktop email client software applications. So is the software application called Incredimail. What is it that you are confused about?*
Wikipedia: IncrediMail Ltd. (NASDAQ: MAIL) is an Israeli company that develops and markets
e-mail client programs and other personal desktop software for the consumer and home user markets.
*[Edit] I think I see the reason for your confusion. You wrote:
My ISP provides an email service, (it not com., but an extension nevertheless, such as org.,net., etc. ) is that a client? overthehill
Your ISP runs an email server somewhere, connected to the internet . They give you a user name, password and an email address and you get the functions that you expect - send & receive emails. That is the "service". Now to use your ISP's email server you need a "client" program on your computer. It sounds like maybe your ISP supplied the Incredimail software for you to use but in fact you will almost certainly find that you may use any email client software application you like.