开发工具 vs2022，新建-控制台应用。项目完整代码下载：

要安装MailKit库

using MailKit;
using MailKit.Net.Imap;
using MimeKit;
using System.Text;
namespace MailKit_mail
{
    internal class Program
    {
        static void Main(string[] args)
        {
            //需要使用 MailKit 库 Install-Package MailKit 
            //注册字符集，缺失字符集，一些中文编码数据为乱码 --千万别删，不然就是乱码了 //参考：https://blog.csdn.net/qq_44384774/article/details/139686615
            Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
            // IMAP服务器的信息
            string host = "imap.139.com";
            int port = 993;
            bool useSsl = true;
            string username = "138********@139.com"; //账号
            string password = "***************"; //密码
            using (var imapClient = new ImapClient())
            {
                // 连接到IMAP服务器
                imapClient.Connect(host, port, useSsl);
                // 登录到邮箱
                imapClient.Authenticate(username, password);
                // 选择INBOX文件夹
                imapClient.Inbox.Open(FolderAccess.ReadOnly);
                // 获取INBOX中所有的邮件
                var count = imapClient.Inbox.Count;
                Console.WriteLine("邮件数量：{0}", count);
                // 获取邮件
                for (int i = 0; i < 20; i++)
                {
                    MimeMessage message = imapClient.Inbox.GetMessage(i);
                    //Console.WriteLine("{0}主题：{1}", i.ToString(), message.Subject);
                    Console.WriteLine($"\r\n标题：{message.Subject} \r\n内容：{message.TextBody}\r\n");
                    // 处理邮件附件
                    foreach (var attachment in message.Attachments)
                    {
                        if (attachment.IsAttachment)
                            if (attachment is MimePart)
                            {
                                var part = (MimePart)attachment;
                                var fileName = part.FileName;
                                //var filePath = "D:\\tmp\\" + Guid.NewGuid().ToString() + fileName;
                                var filePath = "D:\\tmp\\" + fileName;
                                //File.Create(filePath)
                                using (var stream = File.Create(filePath))
                                {
                                    if (attachment is MessagePart rfc822)
                                    {
                                        rfc822.Message.WriteTo(stream);
                                    }
                                    else
                                    {
                                        part.Content.DecodeTo(stream);
                                    }
                                }
                                // 保存附件到本地文件
                                var mailFileInfo = new FileInfo(filePath);
                            }
                    }//附件end
                }
                imapClient.Disconnect(true);
            }
        }
    }
}

运行截图：