How can I reject spoofed mail?

Here is my situation, when I try to send spoofed email of my website like info@mydomain.com to my gmail account it will be rejected due to DMARC policy.

Here is Undelivered Mail Returned to Sender:

This is the mail system at host emkei.cz.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

Host gmail-smtp-in.l.google.com[74.125.206.27] said:
    550-5.7.1 Unauthenticated email from mydomain.com is not accepted due to
    domain's 550-5.7.1 DMARC policy.

But if I try to send spoofed email of my website to the real address:

from info@mydomain.com to info@mydomain.com - the mail will be delivered successfully.

Received: from emkei.cz (emkei.cz [46.167.245.72])

This is my SPF record:

"v=spf1 mx a ip4:myip/32 a:mail.mydomain.com -all"

How can I reject if it's not sended from the same domain?

BTW: I'm running postfix and dovecot.