https://support.microsoft.com/en-au/kb/980534
Or use a different mail client - there are plenty out there.
BTW, Office 365 is brilliant and a bargain in every way - especially if you make use if the family deal for up to 5 users. I've got over a terrabyte of files and photos up in the cloud now and can access them from anywhere on any device. I have the very latest office suite kept up to date and it works fabulously with my mobile devices both on and off line!