mail.arendelle.online vs. arendelle.online/mail?

Let's assume you use "user" to log in to the server, but you use "me" to log in to your e-mail. If you go to https://arendelle.online/mail, you will need to log in again, because the mail server doesn't know "user" as an e-mail user. So, you log in to your e-mail and your cookie will be saved with the new "me" credentials. Then you go back to, for example, https://arendelle.online/video, the server doesn't know you, because there is no user "me" anywhere outside the e-mail section. So you log in again, and so on, whenever you switch from mail to anywhere else. This can become quite annoying pretty fast, right?

So that is why we gave the mail server its own subdomain. You log in to https://arendelle.online/whatever using "user" and you log in to https://mail.arendelle.online using "me", never needing to switch. Because a subdomain uses its own domain cookies, there will be no switching between your accounts. To your browser, https://arendelle.online and https://mail.arendelle.online are considered different domains, each with their own login cookies.

This means that https://arendelle.online.mail uses the same login cookies as any other https://arendelle.online area, like https://arendelle.online/video etc.

https://mail.arendelle.online has its own cookie domain, meaning you can use other credentials to log in to that part of the server.

Why?

Some users have chosen a different e-mail address than their Arendelle Online account, which would cause you to be forced to log in again on https://arendelle.online/mail with your e-mail account, and then log in again in other areas with your other name, which can become quite annoying.

So, IF you are using a different name for your e-mail address, you are advised to use https://mail.arendelle.online so you will remain logged in everywhere.

If your e-mail account is the same as your general server account, you can use whatever you prefer.