1. Configure affected user's mailbox in Outlook in Online Mode (disable cache).
2. Download MFCMAPI tool from http://mfcmapi.codeplex.com
3. In O365 web portal from the Exchange Admin Center, open the affected user's profile, then select Mailbox Features > Mobile Devices > Disable Exchange ActiveSync.
4. Remove the mailbox from the user's mobile device.
5. On the system with Outlook is configured in Online Mode, run MFCMAPI.
6. Click OK to close the Usage Notes screen.
7. Click Session > Logon.
8. Select the Outlook profile set up for the affected user.
9. Double-click the display name of the affected mailbox.
10. In the new window, expand the Root Container menu.
11. Expand ExchangeSyncData menu.
12. For each listed AirSync device, right-click the item, then select Delete Folder and select the checkbox for Hard Deletion > OK.
14. Once all listed AirSync devices have been successfully deleted, from the main navigation select Actions > Exit.
15. Select Session > Logoff.
16. Select QuickStart > Exit.
17. In O365 web portal from the Exchange Admin Center, open the affected user's profile, then select Mailbox Features > Mobile Devices > Enable Exchange ActiveSync.
18. Add the mailbox to the user's mobile device.
19. The issue should be resolved and may be monitored via the Sent Items folder of the configured Outlook.