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Thunderbird can receive email but not send email

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My Thunderbird email (att email) can receive emails but not send them. I get an error message that says "Login to server with [username] failed". Then "Unable to authenticate to Outgoing SMTP server. Check password and authentication method." So in order to use ATT's email with Thunderbird, I recently changed the authentication method to Oauth2. The authentication method for the outgoing server is "Normal password". The other possible settings are Encrypted password, Kerberos and NTLM. Should I be using one of those?

This worked just fine until today. I am puzzled.

My Thunderbird email (att email) can receive emails but not send them. I get an error message that says "Login to server with [username] failed". Then "Unable to authenticate to Outgoing SMTP server. Check password and authentication method." So in order to use ATT's email with Thunderbird, I recently changed the authentication method to Oauth2. The authentication method for the outgoing server is "Normal password". The other possible settings are Encrypted password, Kerberos and NTLM. Should I be using one of those? This worked just fine until today. I am puzzled.

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Use 'normal password' authentication for incoming and outgoing, but enter a secure mail key instead of the regular account password.

https://support.mozilla.org/en-US/questions/1330602