It would be nice if Confluenc would respect the JAVA_OPTS flags that JIRA does to make it easier to restore a full production image and run as a test server.
-Datlassian.mail.senddisabled=true
-Datlassian.mail.popdisabled=true

A workaround I use is to
1) automatically shutdown the server before midnight to present emails going out
2) change the job scheduling file WEB-INF/classes/schedulingSubsystemContext.xml to change the timing of mailPollTrigger to something that will not occur