There are major incoherences in the code that not even the first day developer performs, but surprisingly the coding style is perfect. Obviously that code will not work. It seems you used some AI program to write the Erlang code, right?

There are some XEPs (protocol extensions for XMPP) that describe how such a feature could work:

• https://xmpp.org/extensions/xep-0022.html#sect-idm45549340543024
• https://xmpp.org/extensions/xep-0184.html#example-4
• https://xmpp.org/extensions/xep-0079.html#conditions-def-deliver

Unfortunately, none of those are implemented in ejabberd.

Basically the same functionality that Whatsapp has with the one check mark.

And you plan to accomplish that for free, right?