I am using Virtualmin and I send email using postfix, when we receive bounce back then not show in Inbox but it remain always in Email Queue of Postfix.
This issue in Virtualmin Accure due to lot of incoming(inbound) spam message. due to these messages your spam filter get halted and stop local delivery. If you want to solve this issue follow these steps open Edit Virtual Server >> Enabled features Deselect following features
Not you server not check email for spam but local mail deliver get faster. It's use full only when you receive lot of spam email more then 1000 per hr.