初步完成邮件功能

DMS.Core/Models/EmailMessage.cs

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+using System.ComponentModel.DataAnnotations;
+
+namespace DMS.Core.Models
+{
+    /// <summary>
+    /// 邮件消息实体
+    /// </summary>
+    public class EmailMessage
+    {
+        /// <summary>
+        /// 邮件ID
+        /// </summary>
+        public int Id { get; set; }
+
+        /// <summary>
+        /// 关联的邮件账户ID
+        /// </summary>
+        public int EmailAccountId { get; set; }
+
+        /// <summary>
+        /// 发件人邮箱地址
+        /// </summary>
+        [Required]
+        [MaxLength(255)]
+        public string From { get; set; }
+
+        /// <summary>
+        /// 收件人邮箱地址（多个用分号分隔）
+        /// </summary>
+        [Required]
+        [MaxLength(1000)]
+        public string To { get; set; }
+
+        /// <summary>
+        /// 抄送邮箱地址（多个用分号分隔）
+        /// </summary>
+        [MaxLength(1000)]
+        public string? Cc { get; set; }
+
+        /// <summary>
+        /// 密送邮箱地址（多个用分号分隔）
+        /// </summary>
+        [MaxLength(1000)]
+        public string? Bcc { get; set; }
+
+        /// <summary>
+        /// 邮件主题
+        /// </summary>
+        [Required]
+        [MaxLength(500)]
+        public string Subject { get; set; }
+
+        /// <summary>
+        /// 邮件正文
+        /// </summary>
+        [Required]
+        public string Body { get; set; }
+
+        /// <summary>
+        /// 是否为HTML格式
+        /// </summary>
+        public bool IsHtml { get; set; } = true;
+
+        /// <summary>
+        /// 邮件发送状态
+        /// </summary>
+        public EmailSendStatus Status { get; set; } = EmailSendStatus.Pending;
+
+        /// <summary>
+        /// 发送时间
+        /// </summary>
+        public DateTime? SentAt { get; set; }
+
+        /// <summary>
+        /// 创建时间
+        /// </summary>
+        public DateTime CreatedAt { get; set; } = DateTime.Now;
+
+        /// <summary>
+        /// 更新时间
+        /// </summary>
+        public DateTime UpdatedAt { get; set; } = DateTime.Now;
+    }
+
+    /// <summary>
+    /// 邮件发送状态枚举
+    /// </summary>
+    public enum EmailSendStatus
+    {
+        /// <summary>
+        /// 待发送
+        /// </summary>
+        Pending,
+
+        /// <summary>
+        /// 发送成功
+        /// </summary>
+        Sent,
+
+        /// <summary>
+        /// 发送失败
+        /// </summary>
+        Failed
+    }
+}