Basic Authentication Exchange account

I learned from a previous question I asked that it is possible to use Outlook client on one domain to access an e-mail account on a separate domain.  
My question now is, when setup for basic authentication, the user opens outlook, and is prompted not once, but twice for the A.D/domain credentials for the domain that their exchange server is once.  Both credential requests are for the same exchange server, so shouldn't the user only be prompted one time to access their exchange?  Can someone explain why the same pop up box comes up for credentials twice?
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First for SMTP (mail sending server)
2nd for POP (mail receiving server)
machiavell3Author Commented:
for an exchange account?
If you are talking about RPC/HTTP, then this article explains the difference between basic and NTLM:

Are you using RPC/HTTP?

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