阻止Exchange 2013将邮件正文文本/纯文本转换为HTML

如何解决阻止Exchange 2013将邮件正文文本/纯文本转换为HTML

我正在使用MailKit v2.8从使用IMAP连接到Microsoft Exchange 2013帐户的电子邮件中解析数据。发送到我的Exchange收件箱的邮件正文将100％处于“文本/纯文本”状态。此过程对于新电子邮件来说完全可以正常工作（并且已经投入生产使用了几个月），但是对这些电子邮件的回复/转发大概是在获取时由Exchange转换为HTML的。服务器上回复电子邮件的标头仍然指定邮件正文为“文本/纯文本”。 Outlook还以纯文本显示响应，但是由于某些原因，当我尝试使用MailKit提取消息摘要的TextPart时，它返回null。

MailKit电子邮件提取代码：

using var imap = new ImapClient {
    ServerCertificateValidationCallback = (mySender,cert,chain,sslPolicyErrors) => { return true; },CheckCertificateRevocation = false
};

try {
    await imap.ConnectAsync(_config.ImapServer,_config.ImapPort,SecureSocketoptions.SslOnConnect);
    imap.AuthenticationMechanisms.Remove("XOAUTH2");
    await imap.AuthenticateAsync(_config.ImapUsername,_config.ImapPassword);
    var inBox = imap.InBox;

    if (!string.IsNullOrWhiteSpace(_config.InBox)) { // set inBox to subfolder for devenv
        inBox = await imap.InBox.GetSubfolderAsync(_config.InBox);
    }
    await inBox.OpenAsync(FolderAccess.ReadWrite);
    var uIds = await inBox.SearchAsync(SearchQuery.All);
    var msgs = await inBox.FetchAsync(uIds,MessageSummaryItems.UniqueId | MessageSummaryItems.BodyStructure | MessageSummaryItems.Envelope);

    foreach (var msg in msgs) {
        var bodyPart = msg.TextBody; // <-- this returns null for the latter email,but contains a body for the former
        var body = await inBox.GetBodyPartAsync(msg.UniqueId,bodyPart) as TextPart;
        if (_config.SendingAddresses.Any(msg.Envelope.From.MailBoxes.Select(a => a.Address).Contains)) { // sent from valid address
            // parse and process email body
        } else {
            // discard and expunge
        }
    }
} catch (Exception e) {
    // log exception
}

为简单起见，这是一个示例。使用MailKit提取时，该电子邮件包含TextBody：

Received: from [ExchangeHost] ([ExchangeIP]) by [ExchangeHost]
 ([ExchangeIP]) with Microsoft SMTP Server (TLS) id 15.0.1320.4; Thu,6 Aug 2020
 17:30:12 -0400
Received: from [SenderHost] ([SenderIP]) by
 [ExchangeHost] ([ExchangeIP]) with Microsoft SMTP Server id 15.0.1320.4
 via Frontend Transport; Thu,6 Aug 2020 17:30:11 -0400
IronPort-SDR: [redacted]
X-IronPort-AV: [redacted]
X-AuditID: [redacted]
MIME-Version: 1.0
Message-ID: <[redacted]>
From: <[SenderAddr1]>
To: <[MyExchangeAddr]>
Date: Thu,6 Aug 2020 14:30:03 -0700
Subject: [redacted]
Content-Type: text/plain; charset="us-ascii"
Content-transfer-encoding: quoted-printable
Return-Path: [SenderAddr1]
X-MS-Exchange-Organization-AuthSource: [ExchangeHost]
X-MS-Exchange-Organization-AuthAs: Anonymous
X-GFI-SMTP-Submission: 1
X-GFI-SMTP-Hellodomain: [SenderHost]
X-GFI-SMTP-RemoteIP: [SenderIP]
X-MS-Exchange-Organization-Network-Message-Id: [redacted]
X-MS-Exchange-Organization-Avstamp-Enterprise: 1.0

此电子邮件标题是对上述电子邮件的回复。在MailKit中获取时，它没有TextBody，但是有一个HtmlBody：

Received: from [ExchangeHost] ([ExchangeIP]) by [ExchangeHost]
 ([ExchangeIP]) with Microsoft SMTP Server (TLS) id 15.0.1320.4 via MailBox
 Transport; Mon,10 Aug 2020 11:27:16 -0400
Received: from [ExchangeHost] ([ExchangeIP]) by [ExchangeHost]
 ([ExchangeIP]) with Microsoft SMTP Server (TLS) id 15.0.1320.4; Mon,10 Aug 2020
 11:27:16 -0400
Received: from [SenderHost] ([SenderIP]) by
 [ExchangeHost] ([ExhcangeIP]) with Microsoft SMTP Server id 15.0.1320.4
 via Frontend Transport; Mon,10 Aug 2020 11:27:15 -0400
IronPort-SDR: [redacted]
X-IronPort-AV: [redacted]
From: <[SenderAddr2]>
To: <[MyExchangeAddr]>,<[SenderAddr1]>
Subject: RE: [redacted]
Thread-topic: [redacted]
Thread-index: [redacted]
Date: Mon,10 Aug 2020 15:27:07 +0000
Message-ID: <[redacted]>
References: <[redacted]>
In-Reply-To: <[redacted]>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach:
X-MS-TNEF-Correlator:
x-ms-exchange-messagesentrepresentingtype: 1
x-ms-exchange-transport-fromentityheader: Hosted
x-tm-snts-smtp: [redacted]
Content-Type: text/plain; charset="us-ascii"
Content-transfer-encoding: quoted-printable
MIME-Version: 1.0
Return-Path: [SenderAddr2]
X-GFI-SMTP-Submission: 1
X-GFI-SMTP-Hellodomain: [SenderHost]
X-GFI-SMTP-RemoteIP: [SenderIP]
X-MS-Exchange-Organization-Network-Message-Id: [redacted]
X-MS-Exchange-Organization-Avstamp-Enterprise: 1.0
X-Auto-Response-Suppress: DR,OOF,AutoReply
X-MS-Exchange-Organization-AuthSource: [ExchangeHost]
X-MS-Exchange-Organization-AuthAs: Anonymous

来自MailKit的后一封电子邮件的 HTMLBody：

<html>
<head>
<Meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
<Meta name="Generator" content="Microsoft Exchange Server">
<!-- converted from text -->
<style><!-- .EmailQuote { margin-left: 1pt; padding-left: 4pt; border-left: #800000 2px solid; } --></style>
</head>
<body>
<!-- the stuff that should be in plain text,formatted as HTML -->
</body>
</html>

在Outlook中，后一封电子邮件以纯文本格式设置，就像其标题的Content-Type一样。由于邮件在Outlook中的格式正确，因此我的问题是：

1. 使用MailKit来防止这种转换发生时，我需要做些什么吗？
2. （我怀疑可能性更大），我的系统管理员需要为Exchange帐户设置一些选项来防止这种自动转换吗？

我已经阅读了解决方案here和其他主题，但是由于针对该主题的任何问题都存在近十年之久，所以它们似乎都不适用。

解决方法

1. IMAP（以及MailKit）无法指定不进行任何转换，只是假定不会进行任何转换，因为那不是其他IMAP服务器所做的。
2. 大概在2013年也存在与Exchange 2003相同的选项。

,

因此，我发现Exchange运行正常的原因是因为我出于测试目的而复制了收到的电子邮件。尽管重复的电子邮件具有IDENTICAL标头，但使用MailKit提取Exchange时，Exchange仅将复制的电子邮件转换为HTML。但是，原件正在按预期方式取出。为了将来引用给任何人，如果您希望收件箱保留纯文本，请不要在收件箱中复制/粘贴电子邮件！