Which credential used by send port to communicate with Dynamics ?

I configured my BizTalk application to run on three different host instances: receive, process, and send. I thought the credential used when BizTalk communicates with Dynamics is the Send Handler Host Instance credential. But I got the following error:

The send handler host instance credential was the valid Dynamics user but the process host instance was not. When I switched the credential or changed the credential of process host instance into the valid Dynamics credential, it works fine.

So, is this the way it should work?