Wirelessly posted (my iphone: Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) AppleWebKit/420.1 (KHTML, like Gecko) Version/3.0 Mobile/4A102 Safari/419.3)
While Google allows you to choose to use either protocol with their gmail service, other email providers may not. So, you probably don't have a choice with other email accounts you have and you will need to select IMAP or POP according to what they offer, not which one you want. If you choose a protocol not supported by your email provider, it won't work because of the incorrect settings.