使用MimeKit.LoadAsync加载S/MIME加密邮件时解析消息头失败

使用MimeKit的LoadAsync方法解析S/MIME加密电子邮件，目标是将其转换为MimeMessage对象以便后续解密，但调用该方法时抛出“Failed to parse message headers”错误。未加密邮件测试可正常运行，该邮件发送给自己后能在Outlook中打开并解密，读取内存流也能返回原始消息，编码无问题。

相关代码 #

using var mimeMemoryStream = new MemoryStream(Encoding.UTF8.GetBytes(emailMime), false);
var message = await MimeMessage.LoadAsync(mimeMemoryStream);

邮件内容（敏感信息已移除，加密内容已截断） #

Received: by mx0107p1iad2.sendgrid.net with SMTP id K73DvryGyn Tue, 11 Oct 2022 08:03:25 +0000 (UTC)
Received: from mail-ej1-f46.google.com (unknown [<IP address>]) by mx0107p1iad2.sendgrid.net (Postfix) with ESMTPS id 1100E421485 for <whatever@example.com>; Tue, 11 Oct 2022 08:03:25 +0000 (UTC)
Received: by mail-ej1-f46.google.com with SMTP id d26so22519478eje.10 for <whatever@example.com>; Tue, 11 Oct 2022 01:03:25 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=to:subject:message-id:date:mime-version:content-transfer-encoding :content-disposition:from:from:to:cc:subject:date:message-id :reply-to; bh=cDJY/6ek7SRtGZFiIsYCTyL4DLhWgTozU0FyWoZ++0A=; b=FbnltkWgSgPrKtzIGDaJBHrKTrRfUHUssD1hVejeN9Tpvyy/jSBXos/rSNPvcdjPqqFn9CZR5DBvcrOu4lzknx+c9fythtKdaixkPcEVmNOhZOtla8QsMRApPFoF4smO05LYbJHAKyuQ6tVbe9sL+aOwBptkulVGMpxHGqfS72/5ziuuLumW2cY4QMUCc+y+V3QWPAgU/lInQAQen1Nd02eSibLq072lU0AzPs9/WrwCXbt6N9oi67iPJIn8jPX6FI25ZGR4athf1l050/5I4rlNOITixEn3RhEU95iY/GlkmLYXb76cQ3aHVRMH42dzteftfsgZoFG+MBnJuqQ6Dw==
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=to:subject:message-id:date:mime-version:content-transfer-encoding :content-disposition:from:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=cDJY/6ek7SRtGZFiIsYCTyL4DLhWgTozU0FyWoZ++0A=; b=Orc41aH7O0a4+vODOe/YQs/0+My5X3lskrKdrvRojWWrvvV2zVwjndaSf8ddQ0v8dF8NcaeHVxWBfu7AI+lcrdRiekteiFi0XI+zZwUo2jkH+HtFj+AnZ4FmSFk8HMwI+qgJ4c8xpHTzj/oBRP1EjOQiTptwGtbSKdHROWHbok4ZcQarUHX2+EbEKcLFoDz0TFQLgtvcBRqMFvhNINz0o9aSXC8J9clAoyErqjrQ3zz73ft0mdReuxQnJdzKqwfXOONOEsOgTPka6XP0Qwjub/CZYnEBSlNU+/SkzuQ6WEJGUqEvvYgP6m9Pc6XXZCazgBU+wOBn+IKCG+s+LkrF9w==
X-Gm-Message-State: ACrzQf1oA5iNjn3X5zVuDxGXFNkB1P7tVN/1qcSYtR4d1CZ24JCgvDb8 yJ80uaUDvLgj35jXqBVkqulvWiounXFJziww
X-Google-Smtp-Source: AMsMyM7L+/WNsoR0o2XmR+n4xnKtMdfXqCckvEfwrjXCTDrKifsLhSRtuGtJN0lgyfxYI93UNV8BxA==
X-Received: by 2002:a17:907:2c68:b0:78d:7981:3f8d with SMTP id ib8-20020a1709072c6800b0078d79813f8dmr16356575ejc.724.1665475403776; Tue, 11 Oct 2022 01:03:23 -0700 (PDT)
Received: from [<IP address>] (<domain>. [<IP address>]) by smtp.gmail.com with ESMTPSA id bx10-20020a0564020b4a00b00456cbd8c65bsm8688491edb.6.2022.10.11.01.03.22 for <whatever@example.com> (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Tue, 11 Oct 2022 01:03:23 -0700 (PDT)
From: Sender <sender@example.com>
X-Google-Original-From: "Sender" <sender@example.com>
Content-Type: application/pkcs7-mime; name="smime.p7m"; smime-type="enveloped-data"
Content-Disposition: attachment; filename="smime.p7m"
Content-Transfer-Encoding: base64
MIME-Version: 1.0
Date: Tue, 11 Oct 2022 10:03:20 +0200
Message-ID: <b6d24167-f517-44dc-b21f-bb219cd25e74@mail.dll>
Subject: Encrypted and signed
To: Bob <whatever@example.com>
MIKJpwYJKoZIhvcNAQcDoIKJmDCCiZQCAQAxggHkMIIB4AIBADCBxzCBrjELMAkGA1UEBhMCUEwxHDAaBgNVBAgME0Jp
[truncated]
Tpzn/UAqUMCVY1C/nzU3T0zEcJDxDIb4eaVGr+n6w1Xc2nQs7SOBfXumZ67Ds9fCVeNvk28=

补充说明：读取mimeMemoryStream可返回原始消息，编码无问题；曾将该邮件发送给自己，可在Outlook中打开并解密，未加密邮件测试也可正常运行。

内容的提问来源于stack exchange，提问作者Kuba Chmiel