OSX Mail App suddenly unable to verify account and password?

Ok so randomly tonight, the Mail app on my MacBook Pro is unable to verify my account name and password apparently. I've tried re-entering my password numerous times but to no avail.

I've logged in via the hotmail/outlook website and all is fine, nobodies hacked my account and changed my password etc. So I can't understand why the Mail app is suddenly unresponsive.

Any ideas how to fix?

Delete account and re-add it?

I've got some important emails in my Mail app that I've not deleted and I'd rather not lose them?

When I delete things in the Mail app it doesn't delete them from my hotmail account, and I'm not sure how to retrieve old emails in the mail app as it only seems to show any new emails that are received when linking an account rather than old ones already located in the inbox? If that makes sense.

Ok so I screesnhotted my important email and deleted the account, but now it will not let me re-add the account to Mail because it says it already exists. Although it doesn't.

The only other account there is my iCloud one, which I have no idea what it does or what purpose it serves. That iCloud uses the same email address as the hotmail one I'm trying to add, but obviously I can't delete my iCloud account and it doesn't receive my emails anyways?

Help :/

Check System Preferences > Internet Accounts > Uncheck Mail on iCloud tab.

Check and see if your other account is there. If not, re-add it.

Re-open Mail. If it's still not there, try re-indexing Mail. Details in this link:

http://c-command.com/spamsieve/help/how-can-i-rebuild-apple

Just checked internet accounts, only iCloud account there and Mail is already unchecked.

Only 3 things checked, must be by default because I haven't done them: Notes, Keychain and Find My Mac.

Tried adding my hotmail account "this account already exists" and doesn't let me do it. So I'm still stuck without being able to actually add the hotmail account back to the Mac. I don't get why its all of a sudden stopped working.

Did you try re-indexing Mail?

You can also try removing the Mail cache, and the hotmail folder in the Mail library