I am sorry the preview you're sending of your campaign is showing as from a default address. I checked a couple of campaigns (the most recently sent and on from earlier in March) and found that the from and reply addresses matched in the test to what was selected in the editing of the campaign.
If you could provide a bit more information we can better assist you.
* What is the default address you're seeing?
* What email address are you sending the test to?
* In which campaign(s) are you having this experience?
If you could email communitysupport(at)constantcontact(dot)com with the information requested, we will research and see what we can do to assist you further.