Well after a really helpful reply from GMX support I found a slight mistake in the To: header which I didn't notice except after some RFC5322 reading and manual address parsing:
The To: line contains an invalid "display-name" because the "@" is unencoded, which is invalid (it can appear in a double-quote enclosed string, or must be encoded). Tricky... Big thumbs up to GMX support for pointing out that header line. Now I'll need to find out what broken MUA my colleague is using. Cheers, Hans-Martin 12. März 2021 13:58, "Hans-Martin Mosner via mailop" <mailop@mailop.org (mailto:mailop@mailop.org?to=%22Hans-Martin%20Mosner%20via%20mailop%22%20<mailop@mailop.org>)> schrieb: Am 12.03.21 um 11:53 schrieb Arne Allisat via mailop: Sorry. Me again. Two things: Some people experience temp rejects with an c=irlims message.I thought that is your case as well, but it isn’t. In your case it is an c=hi and you are pointed to the „Sending mail to GMX“ page (https://www.gmx.net/mail/senderguidelines/ (https://www.gmx.net/mail/senderguidelines/)), right? That means one or more of your header are malformed. First three bullets of that page tell you exactly what to do. //Arne As I wrote "as far as we can see, we're not doing anything wrong" :-) Of course I checked the bullet points. * Make sure that your mail server and client time and date are correctly set as specified in RFC 5322 (https://tools.ietf.org/html/rfc5322#section-3.3). This is a postfix system, I'm pretty certain Wietse wouldn't let any code into postfix that does not conform to RFCs. The server has an NTP-synchronized clock as every server should, and of course the dates and times are right (names in Received: header are munged by mailu to protect user's privacy): Received: from authenticated-user (PRIMARY_HOSTNAME [PUBLIC_IP]) by mail.heeg.de (Postfix) with ESMTPA id AF05C7360002; Fri, 12 Mar 2021 09:41:34 +0000 (UTC) Date: Fri, 12 Mar 2021 10:41:34 +0100 * Ensure that the following email headers included in your message are syntactically correct: Date, From, Sender, To. From: User <u...@heeg.de> (mailto:r...@heeg.de) Subject: Re: topic To: =?UTF-8?Q?Recipient_=28recipi...@gmx.de (mailto:=?UTF-8?Q?Recipient_=28recipi...@gmx.de)=29?= <recipi...@gmx.de> (mailto:rene.rze...@gmx.de) Cc: "us...@heeg.de" (mailto:reinh...@heeg.de) <us...@heeg.de> (mailto:reinh...@heeg.de) There is no Sender header, AFAIK it is not mandatory. * Make sure that the following headers included in your email are limited to just one each: BCC, CC, Date, From, Sender, Subject, To. That's the case. No duplicate headers there. As I said, it would be REALLY helpful if the error message included some hint about what is considered wrong. This whole "check whether you did something wrong, here are the rules" when your system already knows or believes to know what I did wrong isn't helpful. If you insist that giving out that information to potential spammers is a dangerous thing then your support should bite the bullet and provide the necessary information when they get a problem report. Cheers, Hans-Martin
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