Office 365 and mobile users

How can we use Office 365 with mobile users must we add their IPs to the Protector?

There is no issue with office365, but the way they use it must be the opposite: they don't have to add O365 as relay server in the Protector, but they must configure O365 to use the Protector as MX server. And they must use the Protector as relay server. This is why it must have a public IP.