Mails get rejected with error code 550 5.4.1 different domains

I've a online tool where different people can create things and other people can book it. So there are three mail adresses involved:

1. My customer: [email protected]

2. The customer of my customer: [email protected]

3. My SMTP: [email protected]

I've no problems with the most of the customers of my customers, but a few of them have pretty hard spam filters and all my mails got rejected.

X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on webout012.mail.myhoster.com X-Spam-Yness: -0.1 X-Spam-Summary: 0.0 HEADER_FROM_DIFFERENT_DOMAINS From and EnvelopeFrom 2nd level mail domains are different

I sent my mails with PHPMailer and my mailer is a simple mail account which sends mails, but all answers end up in very shortlife mailbox.

How can I solve this problem? I googled around but there are just questions about HELO stuff, but that doesn't cares me.

Thanks for any help :)

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