When predefined WCF bindings such as basicHttpBinding, webHttpbinding, and WSHttpBinding are used, the default proxy configuration in Microsoft Internet Explorer settings or in <defaultProxy> of a Microsoft BizTalk Server configuration file are ignored and not used.
This problem is fixed in the following cumulative updates for BizTalk Server:
Cumulative Update 1 for BizTalk Server 2020
Cumulative Update 8 for BizTalk Server 2016
The default proxy is used if the send port is set to "Use proxy" and the proxy address is not provided in the send port transport properties. If "Use send handler proxy settings" is selected, send port uses the same proxy settings as the send handler that hosts this port.
System proxy credentials can be provided from Windows Credential Manager by creating a new generic credential. If there is no generic credential for the corresponding proxy server, the Windows credentials of the context user are used for authentication.
For more 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:
2003097 Information about BizTalk hotfixes and Cumulative Update support