Unable to access Activesync emails when connected to wireless

Hi

I have Linksys WAP54G wireless setup in our office. Mobile devices connected to this wireless network are able to access the Internet but when they tried to access the corporate emails, they received a password prompt to enter their Exchange account password. They are also not able to access their personal emails in their mobile devices. We have verified that Activesync is functional by connecting to a public wireless network and was able to receive emails without any issues.
Please advise how I can resolve this issue.