We had situation where entire mail box was stuck.
Raised SR and Oracle suggested need regular maintenance on smtp mailbox.
1)Sysadmin can create additional Folder say Trash and transfer Processed mails into that Folder
2)If you are receiving mails with Junk characters(eg. Manually forwarded mails) which Oracle can not process that cause seizure of notification processing.
Those mails can be identified in that folder and delete or put in Trash Folder.