I can confirm that this must be a bug with PHP 8.1. I had to switch to 8.1 today because my provider plan to stop support for 7.4 in 4 weeks.
I now also get base64 encoded mails (ascii and html) which are not decoded automatically. Tested in Thunderbird and IOS mail client. Both show the encoded base64 data instead of encoding it automatically.
Email Client is Thinderbird and IOS (both same behaviour), DokuWiki is hosted on a Webspace running Exim.
This is the source of the email:
Return-path: <mxxxxl@scxxxxnn.net>
Delivery-date: Sat, 17 Sep 2022 19:25:42 +0200
Received: from mi016.mc1.hosteurope.de ([80.237.138.239])
by wp365.webpack.hosteurope.de running ExIM with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256)
id 1oZbZW-000489-9v; Sat, 17 Sep 2022 19:25:42 +0200
Received: from wp365.webpack.hosteurope.de ([80.237.133.134])
by mx0.webpack.hosteurope.de (mi016) with esmtps (TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384:256)
(Exim)
id 1oZbZT-0008FJ-OP
for Micxxxel@schxxx.net; Sat, 17 Sep 2022 19:25:42 +0200
Received: from localhost ([127.0.0.1]) by wp365.webpack.hosteurope.de running ExIM with local
id 1oZbZT-00047n-Mb; Sat, 17 Sep 2022 19:25:39 +0200
To: Michael <Mxxx@sxxxn.net>
Subject: [LoxBerry Wiki - BEYO...] Ihr DokuWiki-Passwort
Message-Id: <E1oZbZT-00047n-Mb@wp365.webpack.hosteurope.de>
From: mxxxel@scxxxn.net
Date: Sat, 17 Sep 2022 19:25:39 +0200
X-HE-PHP-Submitted: yes
X-bounce-key: webpack.hosteurope.de;mxxxel@scxxxn.net;1663435539;acebb02b;
X-HE-SMSGID: 1oZbZT-00047n-Mb
X-HE-Spam-Level: /
X-HE-Spam-Score: 0.0
X-HE-Spam-Report: Content analysis details: (0.0 points)
pts rule name description
---- ---------------------- --------------------------------------------------
-0.0 RCVD_IN_DNSWL_NONE RBL: Sender listed at https://www.dnswl.org/,
no trust
[80.237.133.134 listed in list.dnswl.org]
Envelope-to: mxxxl@scxxxn.net
This is a multi-part message in MIME format.
--__________96a3fc930c1cbc8369b65d91dbf67473XX
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: base64
SGFsbG8gTWljaGFlbCEKCkhpZXIgc2luZCBJaHJlIEJlbnV0emVyZGF0ZW4gZsO8ciBMb3hC
ZXJyeSBXaWtpIC0gQkVZT05EIFRIRSBMSU1JVFMgYXVmIGh0dHBzOi8vd2lraS5sb3hiZXJy
eS5kZS8KCkJlbnV0emVybmFtZTogdGVzdApQYXNzd29ydCAgICA6IGtvYmhpaGplYjsxMgoK
[...]
PHP 8.0 shows the same problem, php 7.4 works fine.