Currently if you LDAP enroll a device we attempt to use that user name with the Exchange payload and your regular domain.
The other option we have is if you are using a third party to manage email we added the External Exchange hosted company domain. If you enter in a value here then we would strip the user name which the device was enrolled with and attempt to add the third party domain to that user.
For example if you set the Exchange>External Exchange hosted company domain to myemail.com. Then enrolled a device with the credentials of firstname.lastname@example.org. When you pushed this to the device we would try email@example.com.
As for a fully configurable user option that is different then the enrolled credentials, I have sent this to the product management team and they are going to write a request to add this support.
I marked this as correct since the information is accurate for version 9.5 SP2 Landesk with Mobility. As for the suggested fixes they will not help in this case. If I have an AD logon name of 12345 and an email address of firstname.lastname@example.org there isn't away to configure the Acitvesync settings in AOD to setup email for this scenario.
We will have to manually add each user account to AOD with the correct settings or instruct users to remove the contents of the Email and Username fields and enter their own credentials.