Exchange WebService仅通过c＃发送纯文本

Question

我有一个通过Exchange Web服务发送电子邮件的程序：

//EXCHANGE API
using Microsoft.Exchange.WebServices;
using Microsoft.Exchange.WebServices.Data;

//BODY
string mBody = "<strong>TEXT</strong> Test";

//SETUP
ExchangeService service = new ExchangeService();
service.Url = new Uri("https://"+uriVar);
service.Credentials = new WebCredentials(userNameVar, passwordVar);
EmailMessage message = new EmailMessage(service);
message.Subject = mSubject;
message.Body = new MessageBody(BodyType.HTML,mBody);
message.ToRecipients.Add(recipient);
message.Save();

//Adding Image(s)
string fileFacebook = "facebook.png";
message.Attachments.AddFileAttachment("facebook.png", fileFacebook);
message.Attachments[0].IsInline = true;
message.Attachments[0].ContentId = "imgFacebook";

//Send
message.Send();

（图像添加到原始主体中，而不是在此示例中）

然而，电子邮件以纯文本形式通过，我找不到原因。我很确定它已经工作了一段时间，是否有可能在服务器上发生了某些变化？如果是这样，它会是什么？