something seems to be wrong with the statistics after upgrading to 3.1.6. I sent a newsletter with 3.1.4 to 834 subsribers. The statistics module showed: Total sent 843, Email open 357. So far so good. After the upgrade it shows: Total sent: 0, Emails open 357. Is this a bug?
I probably found the problem. The sent date is sometimes wrong. When I change the date after the newsletter was sent, the “email opened” statistics is reset to zero.