Does anyone know, if I used z-push for a product, would I be liable for not buying a license from Microsoft, for Exchange ActiveSync? It seems they are fairly litigious with activesync patents.
The protocol is patented [0]. If you want to offer your product in the US (and other places that care about software patents), you need a license.
Also note that Z-Push is AGPLv3 software. Unless you can get it under another license from Zarafa, you have to release your product with an AGPL-compatible license.