running a Debian 6 server set up according to the perfect server how to and running the latest ISPCFG3. I have one domain and email domain on my server where a couple of users have complained about something weird and only these users. no other email domain is affected. every now and then, these users will receive an email addressed to their first name @ myservername instead of their real email address and I have no idea why and how this is happening. as an example, the latest one was a user with this email: email@example.com receiving an email addressed to Friederike@h1870666.stratoserver.net h1870666.stratoserver.net being my servername assigned to me by strato. here are the headers of said email: Code: Received: from h1870666.stratoserver.net ([127.0.0.1]) by localhost (h1870666.stratoserver.net [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id trlUW-UJYe39 for <firstname.lastname@example.org>; Fri, 9 Mar 2012 10:08:45 +0100 (CET) Received: from www62.cpt1.host-h.net (www62.cpt1.host-h.net [184.108.40.206]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (No client certificate requested) by h1870666.stratoserver.net (Postfix) with ESMTPS id 95169424127 for <email@example.com>; Fri, 9 Mar 2012 10:08:41 +0100 (CET) Received: from 41-132-49-56.dsl.mweb.co.za ([220.127.116.11] helo=[192.168.2.46]) by www62.cpt1.host-h.net with esmtpa (Exim 4.72) (envelope-from <firstname.lastname@example.org>) id 1S5voK-0000aH-2x for email@example.com; Fri, 09 Mar 2012 11:08:39 +0200 Received: from localhost (localhost [127.0.0.1]) by h1870666.stratoserver.net (Postfix) with ESMTP id 796A44241F4 for <firstname.lastname@example.org>; Fri, 9 Mar 2012 10:08:52 +0100 (CET) Return-Path: <email@example.com> From: "Nigel" <firstname.lastname@example.org> To: <Friederike@h1870666.stratoserver.net>, "SA" References: <4F4CAA58.email@example.com> <firstname.lastname@example.org> <4F4E1AB6.email@example.com> <firstname.lastname@example.org> <4F4F1B8B.email@example.com> <firstname.lastname@example.org> <4F50C15F.email@example.com> <firstname.lastname@example.org> In-Reply-To: <email@example.com> Subject: Re: Baba Indaba Cape Town Show - Premaman Date: Fri, 9 Mar 2012 11:08:28 +0200 Message-ID: <4F59C88C.firstname.lastname@example.org> MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_NextPart_000_0021_01CCFDF4.20BF3070" X-Mailer: Microsoft Outlook 14.0 Thread-Index: AQGnDvqZyJHL3u9+ds0m3sE+F2HtKAFVtdScAXE+HkkCkjPbGAE7mRgYAaGBydcCeqG81QGDw2VKAlZ28Tk= If i try however to send an email to Friederike@h1870666.stratoserver.net it won't be delivered. => The error that the other server returned was: 550 550 5.1.1 <Friederike@h1870666.stratoserver.net>: Recipient address rejected: User unknown in local recipient table (state 14). So far so good but if you have a closer look at the recipient in the above mail example you will see this: Code: From: "Nigel" <email@example.com> To: <Friederike@h1870666.stratoserver.net>, "SA" So the sender must have sent to the proper email address and saved this contact as SA but how on earth did this happen? Is there some Outlook incompatibility happening here? How did Friederike@h1870666.stratoserver.net happen? I am sure the user did not check the email headers and construct her first name @ servername address? any hints? it looks (judging by the headers above) as if the sender is using his providers SMTP servers, that is often the case here in South Africa, most providers even block port 25 but would that cause the recipient to be rewritten?