Very old Outlook email I haven't opened in years

If you used to read that inbox in the Outlook desktop app on an old PC, check that machine before you give up. Outlook keeps a local copy of the mailbox in a .pst or .ost file, so the old emails can still be there even if the online account was closed. Search the old drive for .pst and .ost. A .pst opens in current Outlook with File > Open. An .ost is harder, it's tied to the original profile.

is it an outlook.com / hotmail address? Microsoft closes free accounts that haven't been signed into for a long stretch, so after years there's a real chance it's gone along with the mail in it. try signing in normally first. if it says the account doesn't exist, that's usually what happened. if it just won't take the password, go through 'forgot password' and the recovery form, and fill it from a computer you used back then if you still have one. old subject lines and contacts you emailed help the form a lot.