If you have email addresses and need to get usernames ( this is for the cloud based exchange 365 ) :-
Open powershell and :
Import-Module ActiveDirectory
Also logon to Exchange environment Exchange 2010 in the cloud:-
#Logon
$LiveCred = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell/ -Credential $LiveCred -Authentication Basic -AllowRedirection
Import-PSSession $Session
Then the script is:-
Get-Content emailtest.csv | %{
Get-Mailbox $_ | Select-Object PrimarySmtpAddress, UserPrincipalName
} | Export-Csv users.csv -NoTypeInformation