Exim – Prevent From Spoofing

Hi,

I’m have a problem with default Exim configuration:
User logged in on SMTP as a@a.pl can send mails with spoofed From header, for example from b@b.pl that is other user domain – not owned by a@a.pl

Is it possible to block From header spoofing? Now, SMTP authorization is only theoretical when logged in user (via any account) can send mail as anyone other domain.

Best Regards