customer mails are marked as spam

Hi everybody, i have, again, a problem with the Plesk-Mailing-System...

On my main Domain the Mailserver works fine, but the mail's of the customers with a different domain get lost inside the spamfolders of gmail and hotmail :-(

i have , simply no idea.

Mailheader of a working adress:

-----
Delivered-To: mgskytest@gmail.com
Received: by 10.37.209.201 with SMTP id i192csp2242492ybg;
Tue, 9 Feb 2016 14:24:08 -0800 (PST)
X-Received: by 10.28.102.68 with SMTP id...

customer mails are marked as spam