[exim] Hide sender address in Received header

Hello Mailing List!

I have a complex problem with gmail. I use exim for sending mails from
different users that are connecting over smtp or with a webmailer. Gmail
rejects all mails from my domain with error messages of this type:

550-5.7.1 Our system has detected that this message is likely unsolicited
mail. To reduce the amount of spam sent to Gmail, this message has been
blocked.

I suspect, that Google does not like my Received header because the original
senders ip addresses are not in my SPF record. But I don't understand why this
could be a problem because my exim's address is in the SPF record.

My Received headers look like this:

Received: from localhost ([::1] helo=mydomain.de)
        by vmd4354.contabo.host with esmtp (Exim 4.84)
        (envelope-from <myuser@???>)
        id 1ZBjyF-0004U1-Ni
        for otheruser@???; Sun, 05 Jul 2015 15:28:35 +0200

or

Received: from x5d87bac1.dyn.telefonica.de ([93.135.186.133] 
helo=desktop.localnet)
        by vmd4354.contabo.host with esmtpsa 
(TLS1.0:ECDHE_RSA_AES_256_CBC_SHA1:256)
        (Exim 4.84)
        (envelope-from <myuser@???>)
        id 1ZBjyx-0004UN-77
        for otheruser@???; Sun, 05 Jul 2015 15:29:19 +0200

As a solution I tried is to change the received header by exim. There is the
configuration option received_header_text . I want to change it. But exim
complains >>option "received_header_text" unknown<<.

What can I do? Has anyone an idea for me? Why am I not able to change
received_header_text ? Is it really a problem of my SPF or is Google wrong?
Google is the only provider that rejects my mails.

I hope you have ideas. The mail system seems to me more and more puzzling with
SPF, DKIM. I cannot believe that all server administratos understand what they
are doing.