After installing the Outlook add-on, if it shows (407) proxy authentication error at the time of login SalesHandy credentials in your Outlook (Refer the Screenshot below) then what you need to do is, check whether you have any anti-virus software working on SalesHandy plugin or not.
To check, follow the below steps:
1. Remove SalesHandy Outlook add-on, if installed.
2. Disable your anti-virus software for few minutes.
3. Close your Outlook.
4. Again try to install the Outlook add-on and open your Outlook.
5. Enter your SalesHandy login credentials (Refer the Screenshot below).