Environment
NetIQ Directory & Resource Administrator 8.6 Service Pack 2
Microsoft Exchange 2010
Situation
When trying to clone an user account that has a Microsoft Exchange 2010 mailbox enabled, its fails with the following error message:
"Error performing task in Exchange Management Shell: ExternalMailAddress is mandatory on MailUser. Property Name: ExternalEmailAddress"
Resolution
Prior to DRA 8.1, the property Mailnickname (Exchange Alias) was not populated by default. In order to set the attributed some have used a pre-task trigger script. Since 8.1, both the Account and Resource Management(ARM) and, Directory & Configuration (D&C) consoles set this value automatically, therefore setting it with a pre-task trigger is not necessary anymore.
You should remove or comment out any references to setting the Mailnickname attribution from the pre-task trigger.
Cause
This issue occurs when there is a pre-task trigger is place to set the Mailnickname. When cloning, DRA is backing up properties like Mailnickname in the validation operation and then it puts it back in the pre-task trigger operation. Since the Mailnickname property is set with the trigger before calling the Enable-Mailbox powershell commandlet that creates the mailbox, the commandlet will throw the error.