基于IIS的应用调用Office 365 EWS发邮件遇权限与XML错误求助

我们的应用部署在Windows Server 2019 IIS上，需要通过指定Office 365邮箱账号密码登录，自动向客户端发送邮件，核心是调用EWS接口完成登录并发送邮件。

问题及排查过程 #

• 最初使用EWS官方URL https://outlook.office365.com/EWS/Exchange.asmx，返回**(401) Unauthorized**错误。
  已完成排查：确认邮箱关闭双因素认证、账号密码无误、委托权限配置正确；也尝试了自动发现URL https://autodiscover.outlook.com/EWS/Exchange.asmx，问题未解决。
• 更换URL为https://outlook.office.com/mail/ourdomain.co.za/EWS/Exchange.asmx后，能进入Office 365登录页，但返回The response received from the service didn't contain valid XML错误。

环境信息 #

• 未部署Exchange混合解决方案，仅使用纯Office 365服务，无本地Exchange服务器
• 已排查防火墙规则，未发现请求被拦截
• 尝试在IIS请求筛选中添加并允许目标URL，无效果

当前代码 #

ExchangeService service = new ExchangeService(ExchangeVersion.Exchange2013_SP1);

//Set your Exchange server URL and credentials.
try
{
    // Set the correct URL for Office 365.
    service.Url = new Uri("https://outlook.office.com/mail/ourdomain.co.za/EWS/Exchange.asmx");
}
catch (Exception ex)
{
    return BadRequest("WebService Uri: " + ex.Message);
}


try
{
    service.Credentials = new WebCredentials("****user@ourdomain.co.za****", "****userpassword****");
}
catch (Exception ex)
{
    return BadRequest("Credentials: " + ex.Message);
}

try
{
    EmailMessage email = new EmailMessage(service);
    email.Subject = "Your Visits for today";
    email.Body = new MessageBody("The email is working");
    email.ToRecipients.Add("****Recipients@ourdomain.co.za****");
    email.Send();

    return Ok("Email sent");
}
catch (Exception ex)
{
    return BadRequest("Message Error: " + ex.Message);
}
}

内容的提问来源于stack exchange，提问作者Eddy