Exchange Cached Mode and a full outbox

Came across an awkward issue with an Outlook 2003 client that stopped sending email. All the messages the user was sending were stuck in the outbox. It had been going on for almost two weeks but the user had not realised. The first thing I noticed was that the cached mode of outlook was on – which means an offline copy of the mailbox (an OST file) is created. Knowing that this mode had not been previously active it was discovered that it was active at one time and that the same problem had occurred. The trigger for the offline mode this time appeared to be a migration from an SBS 2003 domain to an SBS 2011 domain – which reactivated it.

It appears that the OST in this case was corrupt or not operating as expected. The next step is to ensure that any mail needed is exported from the mailbox before disabling the cached mode. The import of the data in the pst file can be done after restarting outlook and the messages submitted for sending by placing them back in the outbook and then opening and sending them individually.

However my advice in this case is once you have the data restored and verified and you are sure you dont need the ost file again – move it or remove it to prevent this problem occurring again – and causing your users more problems.