Hi JASM,
Per my knowledge, In Exchange 2010, If you have enabled mailbox audit logging, it can help you to check for user last access:
Understanding Mailbox Audit Logging
http://technet.microsoft.com/en-us/library/ff459237.aspx
If you have not enabled audit logging or you are using Exchange 2007:
You can use this command “Get-MailboxStatistics” to help you check for USER_A LastLogon Time
Get-MailboxStatistics (Exchange 2007)
http://technet.microsoft.com/en-us/library/bb124612(EXCHG.80).aspx
Get-MailboxStatistics (Exchange 2010)
http://technet.microsoft.com/en-us/library/bb124612.aspx
You also need to check pop3 logs and IMAP4 logs to confirm he didn’t use pop3 or IMAP4 to connect Exchange Sever (in the special moth).
Thanks,
Evan