Hi,
Check if CRM URL is added Trusted Sites in Internet Security settings, if not add them.
Check if Internet Security settings are configured for automatic log-in with current user name and password (in Trusted Site), if not change the Security settings.
If above setting are done and MS-CRM outlook client is configured properly then it should not ask the credential again.
If still its happening then problem is in authentication method in IIS.
Follow the below steps to troubleshoot.
1. Go to IIS Manager using inetmgr.
2. Expand tree and Click on Microsoft Dynamics CRM Website.
3. In the Features View, Double Click Authentication.
4. Select "Windows Authentication" and Click "Providers" in right hand pane.
5. Inside Providers dialogue box, Remove all except NTLM and click Ok.
Now your outlook should not ask for your credentials anymore, provided you are logged on same domain in which CRM is installed
Chandan