In order for your incoming emails to be initiated through the service SpamExperts, you need to a) remove the MX records in use and b) add the MX records with the priorities, as received in the activation email. More specifically:

1. Once you have logged into Plesk, click on DNS Settings. You will find all your registered hosting records there.

2. Find the MX records and delete them (cf. image, click to zoom in):

3. Next, add the records and priorities, as received in the activation email:

4. Plesk will ask you to update the changes, so that they can be applied (cf. image, click to zoom in):

5. Once the update is over, the MX records must have the following format: