How to white list IP in Plesk to accept all mails from that IP?
1. Go to Tools & Settings > Mail Server Settings (in the Mail group).
2. Click the White List tab.
3. Click Add Network.
4. Specify an IP address or range of IP addresses from which mail must always be accepted.
5. Click OK.
6. Repeat steps from 3 to 5 to add as many addresses as required.