Mail in Space Missing after Upgrade to 5.1.4

We recently upgraded our system to 5.1.4 from 4.x and we just realized that the Mail that we previously had in a Space is gone!
The index search still shows the mails, but they hyperlink into nothingness.

We did the upgrade a few weeks ago (when we still had the license) by creating a new confluence installation (auto upgrade did not work) and reimporting the last backup. That seemed to work great, but now we see the Mail is gone.

We also DONT have a "Mail Accounts" option in the Space Administration as documented at (https://confluence.atlassian.com/display/DOC/Adding+a+Mail+Account). There is thus no way to re-add mails nor to view old mails.

What can we do ?