Are you still in the old email format or the new one? To tell, look at the URL in the address bar. If it says Zimbra you are in the old one, if it says Appsuite you are in the new one. If you still have the old one, on the Inbox page go to Preferences>>>Email>>>>Email Deletion Schedule. If you are in the new format, there are no schedules you can edit. Messages are kept until you delete them except for the Spam and Trash folders.:
Spam is stored for 7 days or 2,000 messages, whichever comes first
Trash is stored for 14 days or 5,000 messages, whichever comes first
Also, if you are still in the old format, at some point you will be migrated to the new one.