For a MDM project I had to make an dump which users are using which devices to (active)sync with their Office 365 mailboxes. This is how I did that :
Get-Mailbox -ResultSize Unlimited | ForEach {Get-MobileDeviceStatistics -Mailbox:$_.Identity} | Select-Object @{label=”User” ; expression={$_.Identity}},DeviceModel,DeviceOS, lastsuccesssync | Export-csv F:\powershell\activesync.csv
Running the above command showed the following output (in a CSV file format):
As you can see you see the user, the device the user is using, the OS their device uses and last time that devices succesfully synchronized.
I used Excel to import the CSV and sort on LastSuccessSync.
Find out here how to connect to Office 365 using Powershell.