Debian - ISP - Postfix - Helo command rejected: need fully-qualified hostname

Question

When I try to send an email via php mail() function to different domain, it works well. But when I try to send an email to the same domain like domain from where the email is being sent, it does not work and mail log returns this error

Helo command rejected: need fully-qualified hostname (in reply to RCPT TO command))

So it is not working when sending from example.com ( php mail() ) to [e-mail].

/etc/hostame

server

example.com DNS

server.example.com A -> IP of the VPS

PTR

IP of the VPS -> server.example.com

Thank you