Plus adressing issue - email delivered to default account

Hello We have an issue with a customer. email sent to info@domaincustomer.com if tagged as spam is delivered to domainc@domaincustomer.com (default address) in a spam folder as (domainc+spam@domaincustomer.com) and not, as aspected, on folder spam of mailbox info@domaincustomer.com Any idea to how fix this? Thanks