Exchange ActiveSync is a protocol optimised to work with the mobile phone provider's high-latency and low bandwidth networks and enables the mobile phone user to access their Exchange e-mail, calendar, contacts, and tasks even when the mobile phone is offline. WikiPedia has a comprehensive Exchange ActiveSync version comparison table showing the email features supported by a variety of mobile phone operating systems. A small excerpt of this comparison table is shown below.
Of course, Exchange ActiveSync is all about synchronisation. Office 365 enables all supported mobile phones to automatically synchronise email messages, meeting appointments, calendar, contacts, tasks and voice mail using Exchange ActiveSync. The Exchange ActiveSync client allows you to select which information is important for instant download to your mobile phone. To change which information should be synchronised:
Sorting out the client side will fix the majority of email synchronisation issues. However, there have been problems when an older mobile phone OS has been used, for example using earlier versions of the Android OS. If after configuring the user phone there are still synchronisation issues, it would be best to either upgrade to the latest OS and/or examine the Office 365 Exchange setup for mobile phones.
Further information can also be found in the Microsoft Exchange ActiveSync and third party devices knowledge base article.