Get-ExchangeServer | Format-Table Name, *Version*
Get-ExchangeServer | Format-List Name, Edition, AdminDisplayVersion
Here is the list of versions:
Microsoft Exchange Server 2000 6.0.4417
Microsoft Exchange Server 2000 SP1 6.0.4712
Microsoft Exchange Server 2000 SP2 6.0.5762
Microsoft Exchange Server 2000 SP3 6.0.6249
Microsoft Exchange Server 2003 6.5.6944
Microsoft Exchange Server 2003 SP1 6.5.7226
Microsoft Exchange Server 2003 SP2 6.5.7638
Microsoft Exchange Server 2007 8.0.685.24
Microsoft Exchange Server 2007 SP1 8.1.240.6
Microsoft Exchange Server 2007 SP2 8.2.176.0
Microsoft Exchange Server 2007 SP3 8.3.83.0
Microsoft Exchange Server 2010 14.0.639.21
Microsoft Exchange Server 2010 SP1 14.1.218.015
Microsoft Exchange Server 2010 SP2 14.2.247.005
Microsoft Exchange Server 2010 SP2 RU4 14.2.318.4000
Microsoft Exchange Server 2010 SP3 14.3.123.004
Microsoft Exchange Server 2013 15.0.516.032
Microsoft Exchange Server 2013 CU1 15.0.620.029
Microsoft Exchange Server 2013 CU3 15.0.712.024
Microsoft Exchange Server 2013 CU3 15.0.775.038
Microsoft Exchange Server 2013 SP1 (CU4) 15.0.847.032
Microsoft Exchange Server 2013 CU5 15.0.913.022
Microsoft Exchange Server 2013 CU6 15.0.995.029
Microsoft Exchange Server 2013 CU7 15.0.1044.025
Microsoft Exchange Server 2013 CU8 15.0.1076.009
Microsoft Exchange Server 2013 CU9 15.0.1104.005
Microsoft Exchange Server 2016 Preview 15.1.225.17
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The portion of the number after the second decimal is the build number.
Multiple mail servers in your domain?
You can check to see which one you are connected to by version.
Ctrl Click on the Outlook Icon in the Status Bar select connection status.