conjunctions - Can I begin a sentence with, "Unless, that is ...."?

Given a rather long sentence that finishes with something like:

... and therefore, you need not submit that form.

Is it allowable to start the next sentence with, "Unless, that is, you .....", or will this need to be appended to the line after a comma? The reason why I would prefer to start a new sentence is that the first sentence already is rather long and contains several clauses and conjunctions.