We need an add-in (plugin?) for Outlook 2010 that will, whenever an email is being sent, perform the following:
1) Pop up a dialog box that prompts for a customer number
2) Ensure that something has been entered
3) Embed that customer number in X-Header. Example: X-Strata-Plan: 988017
The mail is being sent via Sendmail which will check the validity of the code entered and either send or refuse to send (this is already written and working with Thunderbird).
That's about it.
Please ask anything you are not sure about.