[exim] how to secure alias-overtakings by other mailaccounts

Dear Sir or Madam,

I am using Exim on my server and realized that any user can overtake a
mail address created by an user account. So it seems that for example, I
can just enter an Alias or even change my sender mailaddress for example
in Thunderbird to any mail addresses created by the account.

There is no security check or a warning message like "Hey someone is
using your mailaddress".

How can I secure it? Is there any setting to change this behaviour? That
the owner of the mail address has to agree on using its mail address as
an alias or sender mail address from another account?

Thanks in advance,
Sandra Mende