Best regards

From: =?utf-7?B?UHJvZi5gRHIuIFJ2ZGkgU3NobWllZGU=?= <user@mail.example.org>

this means the string is utf-7 encoded and base 64 encoded:

=?charset?encoding?encoded-text?=

if you consider the encoded text to be base64, you can decode it back to the following:

Prof.`Dr. Rvdi Sshmiede

So him insisting on all his titles makes the MUA to encode it to base64 and then to utf-7 and exceed the maxlenght, which causes a line break

so i guess changeing his sender ID to "Rvdi Sshmiede" without all those titles and encoded chars would totally fix his problems...

edit:

it seems, the MUA aswell malfroms the subject, decodeing the subject from base64 leads to the following:

Testm¡il ÃÌber meine ifs-Adressu

aswell, i see no reason to utf8-encode (first example was utf-7encoded) the sender email, or better said useing  useing special chars in an sender email so it needs to get utf-8 encoded for transmission, i would avoid this anytime just to prevent incompatibilites

Yes, i disabled spamassassin (and almost forgot about it), since my services are behind a relayhost. But it seems the settings are still in use by amavis, which is good.

He needs to change the email client then, it is causing those problems, if common spam filters detect the malformation and some providers even completely reject it, it is an unsolvable problem and not related to iredmail or any software used by it

did you manually disable spamasassin? because normalyl it gets invoked by amavis and only gets bypassed, if you enable spam bypassing for domain or single users

thanks for looking into this ...

554-Transaction
    failed 554-Reject due to policy restrictions. 554 For explanation visit
    https://web.de/email/senderguidelines/(in reply to
    end of DATA command)

I checked all of it, the only useful hint seems to be the header. The user has a not very common email-client. If he shortens his FROM < 50 letters his email will pass, > 50 letters there will be a newline.

cat -v -e ... shows:

X-Virus-Scanned: Debian amavisd-new at mail.my-server.tld$
X-Spam-Flag: NO$
X-Spam-Score: 4.259$
X-Spam-Level: ****$
X-Spam-Status: No, score=4.259 tagged_above=2 required=6.2$
    tests=[ALL_TRUSTED=-1, DKIM_ADSP_NXDOMAIN=0.8, FROM_ADDR_WS=2.999,$
    FROM_EXCESS_BASE64=0.105, HEADER_FROM_DIFFERENT_DOMAINS=0.249,$
    HTML_MESSAGE=0.001, MIME_HTML_ONLY=1.105]$
    autolearn=no autolearn_force=no$
Received: from mail.my-server.tld ([127.0.0.1])$
    by mail.my-server.tld (mail.my-server.tld [127.0.0.1]) (amavisd-new, port 10026)$
    with ESMTP id QD-aK9Sq29VP for <receiver@my-server.tld>;$
    Fri, 19 Nov 2021 15:19:31 +0100 (CET)$
Received: from RS2014 (i6DFA2881.versanet.de [109.250.40.129])$
    by mail.my-server.tld (Postfix) with ESMTPSA id 4Hwf1b58hqz2RJl;$
    Fri, 19 Nov 2021 15:19:31 +0100 (CET)$

From: =?utf-8?B?UHJvZi4gRHIuIFJ1ZGkgU2NobWllZGU=?= <sender@my-serve$
    r.tld>$

Reply-To: sender@my-server.tld$
To: receiver@my-server.tld$
Subject: =?utf-8?B?VGVzdG2haWwgw8xiZXIgbWVpbmUgaWZzLUFkcmVzc3U=?=$
Date: Fri, 19 Nov 2021 15:19:34 +0100$
MIME-Version: 1.0$
Content-Type: text/html; charset="utf-8"$
Content-Transfer-Encoding: base64$
X-MimeOLE: Chaos Intellect v10.2$
x-Account: IfS-RS$
x-MsgStatus: U$
x-linkedname: $
x-ImapUidl: $
X-Mailer: Chaos Intellect v10.3.0.6$

I'm trying to understand why and where the newline actually occurs at first - is it the emailclient or is there a setting in postfix / amavis? Due to RFC 5322 the length of headerfields should work fine up to 78 characters, which would be okay for his original FROM (above is shortend).

the sender gets a bounce message

What's the content of bounce message? It's useful for troubleshooting.

Hi there,

we experience problems with headers containing a FROM like this ...

From: =?utf-7?B?UHJvZi5gRHIuIFJ2ZGkgU3NobWllZGU=?= <user@mail.exampl
        e.org>

The newline in FROM should be okay regarding to RFCs.
The spam-status for those mails:

X-Spam-Status: No, score=4.259 tagged_above=2 required=6.2
        tests=[ALL_TRUSTED=-1, DKIM_ADSP_NXDOMAIN=0.8, FROM_ADDR_WS=2.999,
        FROM_EXCESS_BASE64=0.105, HEADER_FROM_DIFFERENT_DOMAINS=0.249,
        HTML_MESSAGE=0.001, MIME_HTML_ONLY=1.105]

I tried to deactivate the "bad header check" in iRedAdmin-Pro for those users, to get rid of the score for FROM_ADDR_WS, but i didn't work. Spamassasin is inactive, and there is no setting for FROM_ADDR_WS in /etc/amavis.

Anyone an idea?

Thanks in advance