282 votesAndre commented
we had the same Problem and lost about 30 important emails duo to miss synchronisation between public mail accounts and OWA. OWA was not able to act in time so synchronise between to local OST files.
More Over: There was no WARNING "Hey you have more than 500 Folders - STOP Creating Folders"
So we needed two day with microsoftsupport Office 365 Ticket #30126-6846789 to create local recovery files, unable Caching, and reintegrade offline recovery files in the OWA account.
There must be an WARNING, when users have more than 500 Folders!!!!