Setting Up Microsoft Exchange Email Through Gmail

Hi, i have 4 Microsoft Exchange email accounts with Rackspace. Two of my employees work on a voluntary basis and have full time jobs with other companies.

I have their email set up fine on their iPhones but i know need to be able to set up access for them in their laptops. They cannot add additional email accounts on their work computers so i was thinking the only way to accomplish this would be to set their accounts up using Gmail so that they can access and reply to emails through gmail on their desktop... however they must be able to reply from my domain email and not from a gmail address... can anyone help with this please, thanks in advance...