You may receive a notice that the Email Sync needs to be re-authenticated in order to continue composing and receiving emails within your Pipedrive account. This occurs when the login credentials for your synced email account are changed, which breaks the secure connection between your email account and Pipedrive.
To re-authenticate your email sync, go to Tools and apps > Email sync, and click the Stop Syncing button. When reconnecting, click on Add new account rather than Reconnect. This will require you to re-enter your email account credentials, and therefore re-authenticate your email connection.