Hello to all, I have found a solution which allows you to set up multiple addresses that users can send "From". Thanks to the helpful guys at Telstra support.
What you need to do is create a distribution group under Exchange management, in my case two, one for each of the personnel that need to use separate email addresses in the from field. Then as Pratik has suggested you need to use powershell to enable the send as permission for that user.
I have tested it and it works wonderfully. Not being proficient at Powershell, the support guys at Telstra did this for me so I can now view what they have done.
If anyone would like the link to the instructions they sent me then please PM me and I'll send you links, otherwise if you want me to post them here, let me know.
Ian T
Ps Thanks for all the input from everyone here.