Follow the troubleshooting steps below:
1. Check whether the account name and password are correct (You can do so by trying to log in to the account on a third-party client-side).
2. Check whether SMTP/IMAP services have been enabled for the original email account.
How to enable IMAP:
3. Check whether the secure login feature has been disabled.
Log in to the email account and go to Set.
Click Email Link and check whether the secure login feature has been enabled in the security settings. If the feature has been enabled, click Disable secure login.