Exchange 2010 - Licensing

Hi,

We will migrate from Lotus notes 8.5 to Exchange 2010 server ( No possible to use 2013).

We will purchase Exchange 2013 licensing but install Exchange 2010 infrastructure .

Concerning licensing, do you know if we need one Server licensing for each physical server ? ( 1 Cas, 1 for Hub, 1 for mailbox). -> 4 without HA...

Or can we only purchase 1 licensing by complete Exchange role ?

In future, we would like to use DAG ( HA), Do we need also purchasing licensing for server in poassive mode ?

Thanks a lot .

Simon