Unfortunately this is a limitation of Microsoft OMS. When Outlook submits the message to us it does not supply the originating email address, so it is not possible to route the replies back to the originator.
To work around this issue you will need to setup a separate subaccount per user. To access the subaccount page, log into your account on our website and select 'Subaccount' on the left hand side.
Alternatively, you may want to consider using our Outlook SMS Add-in. This offer SMS support within Outlook in a similar way to OMS. Our Outlook Add-in does support reply routing back to the originator, allow multiple users to share the same messaging account.