When adding a subnet in format 203.0.113.* (0 in the middle) in Tools & Settings > Mail Server Settings > White List tab > Add Network it is added with mask /8 instead of /24
It is not possible to remove such subnets from White List in Plesk.
This is a Plesk bug with ID #PPPM-7559 which is planned to be fixed in one of the future updates.
Until the issue is fixed use the following workaround:
- Add subnets to the White list (Tools & Settings > Mail Server Settings > White List tab > Add Network) in the format 203.0.113.0/24 instead of 203.0.113.*
- To clean up Plesk and Postfix from the affected subnets run the following commands (change IP address in the commands to the correct one):
# plesk sbin mailmng-black-white --remove-whitelist --ip-address=203.0.113.0 --netmask=8
# plesk db "DELETE from smtp_poplocks where ip_address='203.0.113.0' and ip_mask=8"