The basic difference is where the email is stored. POP stores on the device (with some provision for eventually deleting from server) in case you can't get on the web. With IMAP you just get a few headers, no copies of email. With web you get nothing locally, you just see a session that can see the mail. From the web server standpoint (the actual question) if the appliance handles all 3, offer all 3 and let the user decide. If maintenance and support is an issue, stick with Web. That way you don't have to answer a lot of stupid user questions like, "Why doesn't this work?" And "I can't find my email!"