It is Data Matrix, Mailmark is basically a logical standard on top of Data Matrix ECC200. Data Matrix allows the blocks to be arranged in multiple layouts, square or rectangular.
Yes, it is accurate to say that all Mailmark barcodes are Data Matrix, but not all Data Matrix are Mailmark. In the document I linked they explain the specifics of the logical standard you mention.
The US Postal Service also uses a DataMatrix on some of their stamps, but not all. I just looked up an official specification document for the USPS codes, but couldn't find it. I did find this page dedicated to postal barcodes. It looks like the USPS ones may more accurately be called IBI Lite?
Can anyone chime in with the security measures present on non-barcoded stamps? I have a Global Forever Stamp on the desk in front of me, and there's nothing that is visible to the naked-eye.
While we're on the subject of barcodes and shipping, check out MaxiCode! I've scanned hundreds of thousands of those little bulls-eyes.
I can't 100% swear to it but I think the postage meter standards in the US are completely owned by the postage meter vendor (Pitney Bowes) and not specified by USPS. These aren't actually "stamps" proper but meter tape. The meter vendor is on the hook to furnish the validation equipment to USPS, so it's all a closed ecosystem to them.
USPS seems to be lax on postage issues... at least some years ago there was some reporting that they very rarely actually scan (and thus mark as used) meter stamps and so you can get away with using them multiple times in practice. I think this is just because of not wanting to spend the money to install the meter validation equipment on sorting machines, since they already have the IMB to use for routing. I suspect they just don't see a lot of actual issues with counterfeiting
Pitney Bowes is much of what I call the "postal industrial complex" - large aspects of USPS technology are completely contracted to Pitney Bowes and a few other companies like Endicia. Pretty much every form of "electronic postage" in the US, including purchased directly from USPS, is managed by Pitney Bowes or Endicia, not USPS (Post Offices used to use an Endicia product for shipping postage, not sure if they still do).