Did you notice? On macOS 10.12 ‘Sierra’, when you try to add a new email account with Mail.app using the guided procedure it seems that you can only have an IMAP one…
Of course you can still have a POP3 account but, like it’s in Apple style in those last years, you have to use a gimnik to achieve the result [if you’re not going Apple-way-of-doing-things be prepared to this].
So let’s see how to do this. Open Mail.app and go to its Preferences, here point to the Account section and try adding a new account… make sure to give a wrong password on pourpose when asked:
After some work behind the scenes, the procedure will say that something is wrong and will ask some more details for this brand new account we’re setting up. As you can see in the following screenshot some more fields to fill will appear, along with a courtain asking the “Account type” we want to set up: IMAP or POP3