For any domain which is using a 3rd party email service, such as Microsoft Office or Google Apps, you would be required to 'disable' the local email service for a particular domain.
1. The first step is to open Plesk and click Domains
2. Click the domain you wish to disable mail
3. Click Mail
4. Click Mail Settings
'activate mail service on this domain'
6. Click OK
to save the changes
7. That's it. You're done!
Here's an interactive tutorial
** Best experienced in Full Screen (click the icon in the top right corner before you begin) **