Thank you for the fast response. However, I was up late last night (again
) and I believe I solved the problem. You see, I had been using the same email software for about 10 years until recently. With this recent upgrade in software, I have learned the real differences in configuring POP3 and IMAP accounts. Now I know that most IP providers have recently (or at least within the last year or two) made IMAP a default setting for the new email software's "wise(z) installers". So, my account was set at IMAP and in IMAP there is no "Delivery" option to be unchecked under the "Advanced" tab. So, I had to reconfigure a new account "Manually" in order to be able to choose a "POP3" setting (which does let you choose whether to leave your mail on the server or not), then delete the first one which was automatically configured as an IMAP.
Would you confirm for me that my conclusions are correct? By the way, what does the seemingly useless IMAP stand for and what is its main purpose or difference that mail servers wanted to make it a default?