External NTLM Auth For Exchange

We recently set up external clients to use NTLM when accessing outlook anywhere. it works correctly and they are able to browse their mailboxes. The problem we are facing is that once outlook opens, they are prompted once to enter credentials and then it goes away after entering. My question is that is this the way it is supposed to work every time they open outlook? I thought the way it was supposed to work was that if it is a domain joined computer trying to access Outlook Anywhere externally using NTLM, the client should send it's cached credentials to the F5 doing Single Sign On. Thus, there shouldn't be a credential prompt.