Unable to add one certain network range to the spam protection whitelist. Other ranges can be added without any problems.
You can see following error when you try to add IP range into whitelist:
MailManager::execWithException(string 'smart_exec', string 'mailmng', array, array, string 'lst')
MailManager->callMailManager(string 'add-whitelist', array)
MailManager->addWhitelist(string 'xxx.xxx.xxx.xxx', integer '16')
The problem in fact that IP range already exists in table smtp_poplocks of sqlite database /var/lib/plesk/mail/poplock/poplock.db. This table has ip_address field declared as unique, therefore it is not possible to add the network with other prefix but with the same IP address, so mailmng process will be failed on attempt to insert this IP range.
As possible solution you can remove record with conflict IP address from /var/lib/plesk/mail/poplock/poplock.db database:
# sqlite3 /var/lib/plesk/mail/poplock/poplock.db
SQLite version 3.3.6
Enter ".help" for instructions
sqlite> delete from smtp_poplocks where ip_address='xxx.xxx.xxx.xxx';
where xxx.xxx.xxx.xxx is your IP address which you can't add as IP range to whitelist.