outlook prompts for password when connecting to exchange

exchange 2003, 2007 or 2010? check authentication methods used by the server and check the client matches these

Here are a few possibilities:
1. If you have a domain, you must logon to your computer with domain credentials in order for you DC to authenticate the user in Exchange.  
2.  Another reason is you have the wrong password saved.  
3. A deeper reason could be the user name could need the domain suffix attached.  like:  mikeG@mydomain.com.

I have performed all of you suggestions.  This issue occurs intermittenly, maybe 3 times a day,  on 4 out of 30 users.

Clark

You may want to take a read at EE member demazter's article below

https://www.experts-exchange.com/Software/Server_Software/Email_Servers/Exchange/A_2300-Outlook-continually-prompting-for-username-and-password.html