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I just added the second level of authentication on email sign in, and when I try to reply to or forward an email, I get "400 bad request". I don't know that adding the authentication is related.
I have closed the app, restarted the computer, shut down and powered back up, and restarted the app; I get one reply, then "bad request".
Can you send a email to yourself? Also, are you using a email client like Outlook or using Webmail?
I was with you until the word "app" then you threw me off a bit. I'm unsure what you had done.
What are you using to send/forward email...webmail or a local email client on your computer?
Two-Factor Authentication (2FA) and OAuth2 are different security standards.
2FA is for an email account on a server...such Cox webmail, Yahoo, Gmail, etc...to verify the person logging into your account is actually you. In addition to the correct username and password, the server wants another factor: token, fingerprint scan, security answer, security code. After you successfully log in via 2FA, this standard wouldn't effect sending/forwarding email. It's just to login.
OAuth2 is for adding said account to an email-client program...such as Outlook, T-bird, Aqua Mail....and doesn't use any of the credentials mentioned above. If you configure your client, OAuth2 uses a third-party website to request a temporary access token from Cox.
If you’re using 2FA, it wouldn't apply to an app...unless the app is a browser.