Customer enabled spam filter for Client side but did not add 'firstname.lastname@example.org' in black list under domain mail configuration at
Domains > example.com > Mail > email@example.com > Spam Filter > Advanced Settings > Black List
After this, 'firstname.lastname@example.org' was added in blacklist for server settings at
Tools & Settings > Mail > Spam Filter Settings > Black List tab > Tools > Add Addresses
Then, server side spam filter was disabled but customer's mail keep filtered.
The issue is known as software issue with id #PPP-10206
The issue will be fixed in further update of the product.