When clicking on the Help -> Kontact Introduction -> Read Manual links it sends you to the following page: https://docs.kde.org/stable5/en/pim/kontact//ontact/index.html This page returns a 404 error.
I don't even see an link for "Read Manual" in the 5.4 version. maybe this has been fixed since 5.2.3
oh, now I see that link. but it does work for me, at least in English. could it be that the translation is broken if you are not using English?
This is going to sound crazy but after rebooting again today the interface is different. The menu is different. The version number hasn't changed (still 5.2.3 which I think is what I opened this as last night) but now that link is gone. Instead there's a link that now just says "KMail has many powerful features that are described in the documentation." When you click on the hyperlink of "documentation" it opens a popup that says "Error -- KIOExec" as the subject and "File not found: help:/kmail/index.html" as the body. I promise I'm not making this up! :) I'm using English under Linux Mint 18.1 KDE version
It was fixed in last version. We moved doc in kmail2 and forgot to update url. You need to update to last version. Regards.