Some time the file mail.que is growing to very large size, you can reduce this size by re-creating the Queue folder on :
C:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\data\Queue
First check if you have some email on queue by executing on exchange powershell console:
Make sure that there is no e-mails on Queue ! Stop the “MSExchange Transport” service -> rename Queue Folder -> start “MSExchange Transport”,
this will re-create the Queue folder.