Enable your incoming mail protection modifyng your Domain MX records to perimeter MX records.
The MX records you need add, are pointing with priority 0, 10 and 20 to perimeter, and with priority 50 to your current mail server if failback is required:

• Set the Mail exchange server with the proper Priority Numbers provided; first of all we will define as, very high(0) exchange server: mx0.perimeter.email, adding the other exchange servers with lower priority.
  • very high(0) exchange server: mx00.perimeter.email
  • high(10) exchange server: mx01.perimeter.email
  • low(20) exchange server: mx02.perimeter.email.
  • if you want add your own excange mail server as failback, you can add it with priority 50. In plesk tipically mail.yourdomain.com: very low(50) exchange server: mail.domain.com.