We currently have an Exchange 2013 setup, which I am in the process of migrating to Office 365, we have a domain which we are going to add for future use for emails. At this moment in time that domain.co.uk has been partly verified (added the TXT records but not the MX records as it may route all live email traffic and cause us to have to do the migration straight away during office hours!) So at the moment only a handful around 4 or 5 users have office 365 business installed, which is connecting to our on-premises Exchange 2013 server which is fine.
The problem comes when users connect via our Watchguard XTM525 firewall from home which logs them in fine, but as soon as they load up their Outlook they get a credentials box pop-up which not only asks for there own email credentials such as bob@domain.co.uk but then also asks for dean@domain.co.uk.
Is there any way in which I can stop the credentials boxes from popping up ? We are also looking for a cloud only deployment not a hybrid server deployment.
Thanks