re set an email password
Unable to re establish an e mail account where we had to change a password. All other apple products no problem. Just the old Mac. Help
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Mail itself does not require a password; individual account do.
If an account is not working, remove it and recreate the account from the details supplied for such by the web mail service.
Add email accounts in Mail on Mac - Apple Support
You don't change a password using mail. All you do is enter the password you created when setting up the email account on the website of the service in question. Yahoo, gmail, proton mail and etc.
Mail basically mimics the web service. The web service is the source for creeated passwords, outgoing mail server, incoming mail server and etc,