This command will give you a .cvs with user names and the last date and time of login. See KB1039659 to get started.
After getting into the server run this command. There is a catch: you need to change the file path or on the root of C you could just create a folder called ‘ out ‘
Get-mailbox -resultsize unlimited| Get-MailboxStatistics | select displayname, lastlogontime | Export-Csv C:outLastLogin.csv