You tried to move some GW mailbox between two Post Offices, but it fails with the D07E move error status. The mailbox does not appear in a desired destination location, nor any mails were transferred.
The D07E move error is a duplicate entry problem. A move process detected that if you move such a mailbox into a destination PO, this will create a duplicate entry that slashes with existing user object in the PO.
If you check from gwadmin console -> System -> Pending Operation, this account would show in a pending status. Undo pending from this place.
Next, go to a user account itself and check properties -> More -> User Move (that option will be listed on a user object if the move is pending or not yet finished). From Move status, undo the D07E error by clicking on Clear Status. Wait till this change replicates and the user shall be then in original status before the move.
Most likely, this is caused by email addresses and allowed formats in a source and destination POs. For instance:
1. In PO1 a GW user "student123" had names Jan Holger. In a PO1 allowed Internet Addressing formats do not list First.Last / Last.First versions, only userid@domain.
2. In PO2, a desired destination PO, there is a user "student456". He also has assigned a name Jan Holger alike student123 in PO1. On PO2 you have enabled First.Last and/or Last.First Internet addressing formats.
3. If you moved student123 to PO2, he will become the same, conflicting email address originating with the First.Last allowed formats under this PO. And that is why a move is not allowed.
This was just a simple example but gives you an idea what place to look at and see what email address the moved account would get assigned as allowed in a new PO and if that would conflict with any existing user.
Sometimes neither that can help. In a case of GW user having some eDir or AD associated counterparts, you might want to disassociate those accounts and re-try moving the account as solely GW one.
If that does not help, consider renaming MailboxID via More -> Rename option on a user GW account. When renaming it, avoid reserved characters like spaces. Use anything like JHolger-1.