This bug is in Zimbra Suite, I tried to post on "Zimbra Mobile Plus" forum but no one seems to be there, so I try again here.
https://forums.zimbra.org/viewtopic....274555#p274555

I converted an NE to OS, and installed Zimbra Suite Plus on it.
Now, when I send a reply from mobile, the email is not shown in the same thread as the original email, I checked the "thread-index" field and it's different.

With NE, I didn't have this problem.

Checked with a Samsung S6, the EAS version is 2.5 (default for samsung mobiles, in zsp console).

Thanks
Stefano