Contact check - is user from contact book registrated

Hi all,

after getting access to contact, i want to do the next step.

Now I’m looking for something like a contact check like whatsapp or any other messenger app.
I just want to check, if the users from the contact book are registrated at my site (just a check, if email-address is signed up).
I know that whatsapp received many critic just because how they work with the user contact book.

So my question is:
What is a good way to check this? Is there any best practice example?

Anyone an idea?

Hey were you able to resolve this?

Nope, but i will working on it the next days/weeks