How can I get rid of "empty subject header" message in Yosemite Server?

Hi

When I get a mail without an subject text, the sender gets a Message like:

said: 550 5.7.1 empty subject header (in reply to end of DATA command)

I found, that I should change /etc/postfix/custom_header_checks to deliver even empty subject mails.

which I have done, but it doesnt work.

Yes, I also had the idea, that it must be done in the Dovecot.config, but I cannt find a howto...

Any help for me, howto do it or where to find an answer?

Thanks from Berlin/Germany

Al