Sorry for the delay.
We solved it by going to http://bit.ly/1cloNVO to make the object not 'syncable' which it will make it to be filtered.
User objects are filtered if:
mailNickName starts with "SystemMailbox{"
and also
msExchRecipientTypeDetails == (0x1000 OR 0x2000 OR 0x4000 OR 0x400000 OR 0x800000 OR 0x1000000 OR 0x20000000)
Thus solving the error.
You can get more details of List of Attributes Synchronized to Office 365 Tenant from: http://bit.ly/Is04nU