BizTalk Server runtime does not use the authentication credentials that are configured in the system web proxy settings.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
This problem is fixed in the following cumulative update for BizTalk Server:
Cumulative Update 8 for BizTalk Server 2016
Note When a system is configured to use a web proxy with authentication, BizTalk will first look for the proxy credentials in the Windows Credential Manager. These credentials should be added to the Generic Credentials section of the Credential Manager in the Control Panel. If no credentials are found for the proxy server, BizTalk will use the Windows credentials. Host instances need to be restarted each time credentials are added.
For information about the service packs and cumulative update list for BizTalk Server, see the following article in the Microsoft Knowledge Base:
2555976 Service pack and cumulative update list for BizTalk Server
For more information about BizTalk Server hotfixes, see the following article in the Microsoft Knowledge Base:
2003907 Information about BizTalk hotfixes and Cumulative Update support