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.

[0] http://www.microsoft.com/en-us/legal/intellectualproperty/IP...