Plesk allows us to add IPs to its anti-spam filter to allow or block email from these IPs. We use Whitelists and Blacklists. If we add an IP to the Whitelist, the email service does not filter the email that comes from this IP. On the contrary, in the Blacklist, a domain is indicated, not an IP, and the email service blocks any email that comes from this domain.

To add IPs to the whitelist and not block any email from this IP, we have to access our Plesk panel. Then we go to Tools and Settings > Mail Server Settings.

When we access it, we see a series of tabs above. Click on the tab that says Whitelist.

Here we see a list of the IPs we have on the whitelist. To add a new IP, click on Add Network.

Here we must indicate the range of IPs we want to add to the whitelist. If it is a single IP, we must indicate /32 at the end of it, for example:
We want to add the IP 1.2.3.4 to our whitelist. We must indicate in the field 1.2.3.4/32

When it is indicated, click on the OK button and in this way, the IP would be on the whitelist of the email server.

Add domains to the blacklist

To add a domain to the blacklist, we access our Plesk panel. Then we go to Tools and Settings > Mail Server Settings

In the window that appears, we have a series of tabs above. Click on Blacklist.

When accessing it, it will show us a list of the domains that we have blocked. To add one, click on Add domain.

In the field, we indicate the domain we want to block, and then click on OK. With this, we will have the indicated domain blocked in our email service.