Hi Guys, we have a version working in our ZCS 8.8.9, we found it in https://github.com/Zimbra-Community/zimlets-foss
Download the project and extract the zimlets-foss-master.zip\zimlets-foss-master\Zimlet\src\zimlet\com_zimbra_undosend, zip again the files to com_zimbra_undosend.zip
Paste in /opt/zimbra/zimlets/ and install (:~$ zmzzmzimletctl deploy com_zimbra_undosend.zip), the version is the 2.0
P.S.: Thanks to kkto for you comment in https://www.zimbra.org/extend/items/view/undo-send