您可以使用下面的Microsoft Graph API查询来读取特定用户的邮件。

GET https://graph.microsoft.com/v1.0/users/username@xxxxxxxxxx.onmicrosoft.com/mailFolders/inbox/messages

我注册了一个Azure AD应用程序，添加了API权限，如下所示：

现在，我通过Postman使用客户端凭证流生成了一个访问令牌，参数如下：

POST https://login.microsoftonline.com/<tenantID>/oauth2/v2.0/token
client_id:<appID>
grant_type:client_credentials
client_secret:<secret>
scope: https://graph.microsoft.com/.default

答复：

当我使用上面的访问令牌运行Graph API查询时，我在response中成功获得了邮件列表，如下所示：

GET https://graph.microsoft.com/v1.0/users/username@xxxxxxxxxx.onmicrosoft.com/mailFolders/inbox/messages
Authorization: Bearer <token>

答复：

要在c#中运行相同的Microsoft Graph查询，可以使用以下示例代码：

using Azure.Identity;
using Microsoft.Graph;

namespace GraphApp
{
    class MSGraph
    {
        static async Task Main(string[] args)
        {
            var scopes = new[] { "https://graph.microsoft.com/.default" };
            var tenantId = "tenantID";
            var clientId = "appID";
            var clientSecret = "secret";
            var options = new TokenCredentialOptions
            {
                AuthorityHost = AzureAuthorityHosts.AzurePublicCloud
            };
            var clientSecretCredential = new ClientSecretCredential(tenantId, clientId, clientSecret, options);
            var graphClient = new GraphServiceClient(clientSecretCredential, scopes);
            var inbox = await graphClient.Users["userID"].MailFolders["Inbox"].Messages.GetAsync();
        }
    }
}

参考：Build .NET apps with Microsoft Graph