dovecot - Mar 2015 - Strange empty folders 1 1 1 1 1

I just updated to the latest dovecot from 1.2 and it has been working great
for most of my clients. But one client,who is using Outlook, has found that
hundreds of folders have been created, e.g.

admin at domain Deleted Items 1
admin at domain Deleted Items 1 1
admin at domain Deleted Items 1 1 1
....
admin at domain Deleted Items 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

This appears to be affecting all folders in his account, across multiple
accounts for his domain. It isn't affecting any other user, even other
users using outlook.

All the folders are empty.

Just wondering if anyone has any suggestions or ideas why this is happening.

Thanks
Samuel