Question: HC Panel throwing following error while activating BlackBerry User.
 Error: 72304140 : Error occurred while activating BlackBerry user. BES Server cannot find Exchange MailStore User.

Answer:

Using BlackBerry Xpress version 5 normally it take sometime to pull information of newly added user from the Active Directory.

To update immediately the new user from Active Directory in the BlackBerry Enterprise Server 
Open BlackBerry Administration Service > Servers and components > BlackBerry solution topology > BlackBerry Domain > Component View > Email... on right side screen click on Refresh available user list from company directory.

See below the screenshot