Be sure to close out of Outlook before beginning the process below.
To remove previously cached/saved credentials on your workstation using the Windows Credential Manager under Windows 10, perform the following steps:
Press the Windows key on the keyboard or click the Windows Start icon.
Start typing Credential Manager, and select the Credential Manager icon.
On the resulting screen you will see the choice to manage your Web Credentials or your Windows Credentials.
Delete any credentials under the 'Windows Credentials' grouping that is associated with your AppRiver email account, . To do this, click on the down arrow associated with the saved credentials and if you see an entry with "Office 15\" or "Office 16\" and your username, choose the option to 'Remove'.
The example above may be different from what you see. You only need to remove the email address entries associated with AppRiver. Then you will re-start Outlook, remove anything in the username and password field, then re-enter your email address and password.
If you are still experiencing the issue please contact Support by chat, email, or phone.