I've installed Allocatus Server and configured to access Exchange Server. I'm receiving the following error message: An attempt to access the mailbox of the Domain\UserName was not successful. Either the credential of the service account is invalid or the authentication method used is not correct. What's wrong?

Make sure you have setup a Windows account with an Exchange mailbox and you provided valid credentials in the Allocatus Server Configuration dialog. Also verify the authentication method (see http://community.allocatus.com/forums/thread/416.aspx)