After upgrading some users to Office 365 x64 we ran into a consistent issue. Each user, upon boot up would receive the following error.
Microsoft Office Outlook
Either there is no default mail client or the current mail client cannot fulfill the messaging request. Please run Microsoft Outlook and set it as the default mail client.
It seems when upgrading or changing the version, either registry entries or traces of the old version get corrupted. Links from 3rd party programs to not properly update to the latest install. Even after setting Outlook as the default mail program. Adobe Reader is a common culprit to produce the above error on startup.
Download the Office Removal Tool – At this time, it is found here: Office Removal Tool
You can also typically find it by Googling: Office 365 Removal Tool.
Run the tool -> restart -> reinstall version desired and rock out!