Configuring External mail domains to send from Company mail Server

We have a number of Field Reps that do not have company mail accounts.  The main reason for this is that we dont provide phones, they use their own and we dont want to configure our mail settings on their phones.  We're a PCI company and can't let people use their own devices on out mail server.

What are the negatives to creating their personal mail accounts as 'mail contacts' on our mail server?
What are the other option available to me for doing this?