Learn English – How to use “due date”

Here's a question that the Google NGram Viewer can actually help with. Look at the values on the Y axis. The usage of "Due to" at the beginning of a sentence has reached 0.0008% in 2008. That may not sound like much, but it represents orders of magnitude more frequency than most of the NGrams offered as proof on this SE site.

Only the severest and most arbitrary prescriptivist would argue that all these instances are wrong.

I think binding (which, according to NOAD, means impose a legal or contractual obligation on) is the more fitting word here. Mandatory simply means something is mandated (such as a mandatory helmet law for motorcyclists); binding implies that something (such as a date) has been agreed to by both parties.

As for alternative suggestions, you might consider contractual deadline, but I'm no expert in contract law. A lawyer could probably give you a better answer.