Error '407 proxy authentication required' is popping up when using Salesforce for Outlook

Note: Information in this article applies to both Classic and Lightning Experience

This error is received when a proxy server with a basic authentication is configured in Internet Explorer. It can be resolved by not using a proxy or adding the following URL in the proxy server setting

salesforce.com
*.salesforce.com
*.force.com
*.login.salesforce.com
 

Check the following in the Internet Explorer options

1.  Close all your applications and then press and hold down the Windows key on your keyboard and press the R key key to bring up the Run command box. Type in inetcpl.cpl and press Enter to open up the Internet Explorer Options

2.  Click on the Security tab and click on Trusted sites | Sites and make sure the 'https://*.salesforce.com' is added in there if not add it and click on Close

3. Next click on the Connections tab and click on LAN settings. On the Local Area Network (LAN) Settings page, under Proxy server, do they have "User a proxy server for your LAN" checked off?  If YES, then you may need to ask your IT team to bypass the URLs above. 
 

- Under Proxy server section, next to where the Address and IP address is typed in click the Advanced button.
- Under Exceptions, type in the following:
 
salesforce.com;*.salesforce.com;*.force.com;*.login.salesforce.com

- Click Ok and then Ok again.

4. Restart IE and then restart SFO and then the login screen should come back up. If not, right click on the SFO icon in the System Tray and click on Settings