Great news for Exchange Online organizations, the message trace will be extended to 90 days!
Message trace enables administrators to trace email messages as they pass through Exchange Online or Exchange Online Protection (EOP) service. It helps you determine whether a message was received, rejected, deferred, or delivered by the service.
You’ve told us that you need to be able to trace messages older than the current period of one week. We’re excited to announce that we’re increasing the look back period for message trace in Exchange Online to 90 days!
Read more at source: http://blogs.technet.com/b/exchange/archive/2014/02/04/extending-message-trace-to-90-days-in-exchange-online.aspx