Hello,
it seems that the problem has not been completely solved.
If I send a mail to a t-online user, I do not longer receive any errors, but within the plesk overview at (/server/mail_queue.php), there I can see that mail.
The details are:
X-No-Relay: not in my network
Received: from [127.0.0.1] (unknown [xxx.x.xxx.xx])
by xxxxx.xxxxxxx.com (Postfix) with ESMTPSA id CxxCAxxxxxxx
for <xx-xxxxxx@t-online.de>; Fri, 8 Mar 2013 15:25:49 +0100 (CET)
Message-ID: <5139F4ED.3080504@xxxxxxxxxx.com>
Date: Fri, 08 Mar 2013 15:25:49 +0100
From: "xxxxxxxxxx.com" <shop@xxxxxxxxxx.com>
Reply-To:
shop@xxxxxxxxxx.com
Organization: xxxxxxxxxxxx
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:17.0) Gecko/20130215 Thunderbird/17.0.3
MIME-Version: 1.0
To:
xxx-xxxxxx@t-online.de
Subject: Order number 4500012
Content-Type: multipart/alternative;
boundary="------------020109030802060303090501"
I do not know what the number CxxCAxxxxxxx is standing for, therefore I have hidden it for security reasons.
I hope that someone can help me with that.
Thank you in advance.
Edit: SOLVED. I have just contacted the abuse department of t-online and they replied me that someone with a similar IP-Address has been blacklisted for spam. Therefore mine was also blocked. But they will release my IP now.