As the "MailNickName" is an exchange attribute, it is handled specially by the DSA and skipping this from the domain pair properties | object matching | Attributes to skip does not help.
Modify the following registry key on the DSA agent host.
Name: [HKEY_LOCAL_MACHINE\SOFTWARE\Aelita\Migration Tools\CurrentVersion\Components\MBRedirector]
String value: SetMailNickname = 0
Note the Key on 64bit systems is being HKEY_LOCAL_MACHINE\Software\Wow6432Node\Aelita\Migration Tools\CurrentVersion\Components\MBRedirector]
Quest does not provide support for problems that arise from improper modification of the registry. The Windows registry contains information critical to your computer and applications. Make sure you back up the registry before modifying it. For more information on the Windows Registry Editor and how to back up and restore it, refer to Microsoft Article ID 256986 ‚Description of the Microsoft Windows registry: http://support.microsoft.com/default.aspx?kbid=256986
Important: This change will affect mailbox enabling of target users and needs to be implemented after all target users were successfully mailbox enabled. Please see KB SOL24845 - https://support.quest.com/migration-manager-for-ad/kb/24845
“11) RUS is processing system Mailbox Enable User policy. The filter that is placed on this policy by Exchange setup will match any user who has both mailnickname and any one of either homeMDB, homeMTA, or msExchHomeServerName attributes set. Since there are two attributes required by the Default policy exist right after the DSA has processed the object (mailNickName and homeMTA or homeMDB), this policy works and RUS completes mailbox enabling of target user via populating missing Exchange attributes.”
Skipping Mailnickname via registry will still let the rest of DSA’s mailbox-enabling steps to happen. HomeMDB, HomeMTA and LegacyExchange DN will be set on the target accounts. However before Mailnickname is set, they are not going to be picked by RUS and mailbox enabled. There is a possibility to run some sort of VBS script that would look at each object’s mailnickname, and for those where it’s missing while HomeMTA or HomeMDB are present, populate mailnickname from other attribute, for example from SamAccountName.