The first question to ask is, does any user that has Windows Mobile able to connect without issues? If no, then Exchanges ActiveSync is setup correctly. If not, then your Exchange server is not ready for mobile connections yet...
If exchange is working with a Windows Mobile device (or has worked before):
1. Under ADUC, open EACH user that needs access to email via their mobiles. Under the Exchange Features tab, ENABLE all Mobile Services. (Outlook Mobile Access, User Initiated Sync, and Up-To-Date Notifications)
2. Under ESM, go to Global Settings, right click on Mobile Services, and go to properties. Just for a test, put a check mark in every box on the General tab. Go to Device Security now, and uncheck Enforce password on device. (these settings are not secure, but at least we will be able to test if exchange is truly setup for mobile access)
3. Under ESM, go to Recipients, under "Offline Address Lists", right click on "Default Offline Address List" (you may have other ones, but you should at least have the Default one here), click on "Rebuild". Depending on the size of your Active Directory, you may have to wait 5 minutes, 10 minutes, or even and hour. (usually 5-10 is average for a small-medium size business). This will force a rebuild of all the AD information into exchange, which speeds up the changes we made in ADUC earlier.
Now, after you have waited 5-10 minutes (or longer), try refreshing your droid again. As long as exchange's activesync is setup correctly, you should be able to start receiving emails. If not, check your log's on the exchange server. (check IIS logs for connections to see if your user's mobile is indeed trying to contact the server, check security logs to see if the password/user/domain is correct, check exchange logs for errors in settings or access attempts)
If none of this works, then my advice is to go through and check/reconfigure your exchange for mobile access (activesync). Most of this will be done through the IIS manager. (most likely wrong settings somewhere)
And lastly, the biggest problem I have seen are SSL certs that are self signed, or not configured correctly. I highly recommend checking your SSL certificate for accuracy. (or if self-signed, have your intermediate certs installed onto the droid device)
Cheers