Really quick one here, if you would like to find out when a user last logged onto Exchange 2013 to check their email then we can use the following command in power shell:
Get-MailboxStatistics -Server "mailboxserver.domain.com" | Sort LastLogonTime -Descending
You will need to change the server name to be the server that the users mailbox was located. If you would like a lot more detail such as the IP, endpoint and client that was use to log onto this mailbox then mailbox auditing will need to be enabled. To enable mailbox auditing in Exchange 2013 then check out this link.
http://technet.microsoft.com/en-us/library/ff459237(v=exchg.150).aspx
This will also apply to Exchange 2010.