I can tell you from a technical perspective the product will work without additional licensing. However, you should check with Sales for a ruling on the actual policy. I recall EmailXaminer required the purchase of additional licenses given a similar scenario. I can't tell you for sure how this should be handled.

Good question. I found an document which might not reflect current state, but here it goes:

EmailXtender (EX) is licensed on a per mailbox basis. Pricing for the mailboxes is a tiered 
structure and only one EX tier should be used per quote. There is no minimum mailbox 
requirement.
 
EmailXtender Archive Edition is licensed on a per mailbox basis. EmailXtender Archive 
Edition is a subset of the EmailXtender functionality. The entry level and minimum mailbox 
licensing is 50 mailboxes. When over 50 licenses are needed, supplemental licenses are sold
in units of one.
 
EmailXaminer is licensed on a per reviewed email mailbox basis. It is licensed on a named 
user basis and the named licenses cannot be re-assigned to different named mailbox users 
even after the departure of the employee. An EmailXtender license is required. There is no 
minimum mailbox requirement.

The question now is what mailbox is considered to be licensed here - managed on Exchange or "extended" one? That would be valid question as I recall one discussion few months ago where I was that when licenses is expired (or removed) archiving will work, but calling it back won't. I do not recall was that said for DX or EX. But given that I have no clue about EX and DX your best bet is presales.

Well the upper line seems pretty clear, right?

As I said, I don't know much about EX world (except it exists). Above quote you were referring to is for EmailXaminer so if that make it clear for you then fine

From the EMC Case:

Licenses are related to the active Exchange users who can search archived emails; everyone who can use Outlook and Search Plugin are accounted in this number.