If you are receiving the red error message in the upper right-hand corner of your screen that states 'Could not verify access credential', this typically means that something has changed with your email account on the email client provider end. In most cases, this occurs if you have changed the password you use to log in to your email client provider.
In these instances, the steps are quite simple to alleviate the error from occurring and moving forward with sending emails.
From your Settings, simply click on Email & Phone, then Email Accounts. From there, simply click below on your email client provider, and proceed through the OAuth process.
Please check out the short video below to see an example of re-authenticating your email address!