SmtpClient发邮件时为什么用MailMessage.From而不用MailMessage.Sender

SmtpClient sc = new SmtpClient();sc.Host = "邮箱服务器地址";MailMessage mm = new MailMessage();mm.Sender = new MailAddress("公司邮箱", "发件人");mm.To.Add(new MailAddress("我的163邮箱", "接收人"));mm.CC.Add(new MailAddress("抄送的邮箱", "抄送人"));mm.Subject = "测试程序发送邮件";mm.Body = "<b>我发送了邮件---我是程序发出的</b>";mm.IsBodyHtml = true;mm.PRiority = MailPriority.High;sc.Send(mm);

sc.Credentials = new System.Net.NetworkCredential("发件邮箱", "密码");

mm.From = new MailAddress("公司邮箱", "发件人");

public void Send(MailMessage message)        {            if (Logging.On)            {                Logging.Enter(Logging.Web, this, "Send", message);            }            if (this.disposed)            {                throw new ObjectDisposedException(base.GetType().FullName);            }            try            {                if (Logging.On)                {                    Logging.PrintInfo(Logging.Web, this, "Send", "DeliveryMethod=" + this.DeliveryMethod.ToString());                }                if (Logging.On)                {                    Logging.Associate(Logging.Web, this, message);                }                SmtpFailedRecipientException ex = null;                if (this.InCall)                {                    throw new InvalidOperationException(SR.GetString("net_inasync"));                }                if (message == null)                {                    throw new ArgumentNullException("message");                }                if (this.DeliveryMethod == SmtpDeliveryMethod.Network)                {                    this.CheckHostAndPort();                }                MailAddressCollection mailAddressCollection = new MailAddressCollection();                if (message.From == null)                {                    throw new InvalidOperationException(SR.GetString("SmtpFromRequired"));                }                if (message.To != null)                {                    foreach (MailAddress current in message.To)                    {                        mailAddressCollection.Add(current);                    }                }                if (message.Bcc != null)                {                    foreach (MailAddress current2 in message.Bcc)                    {                        mailAddressCollection.Add(current2);                    }                }                if (message.CC != null)                {                    foreach (MailAddress current3 in message.CC)                    {                        mailAddressCollection.Add(current3);                    }                }                if (mailAddressCollection.Count == 0)                {                    throw new InvalidOperationException(SR.GetString("SmtpRecipientRequired"));                }                this.transport.IdentityRequired = false;                try                {                    this.InCall = true;                    this.timedOut = false;                    this.timer = new Timer(new TimerCallback(this.TimeOutCallback), null, this.Timeout, this.Timeout);                    string pickupDirectory = this.PickupDirectoryLocation;                    switch (this.DeliveryMethod)                    {                    case SmtpDeliveryMethod.Network:                        goto IL_235;                    case SmtpDeliveryMethod.SpecifiedPickupDirectory:                        break;                    case SmtpDeliveryMethod.PickupDirectoryFromIis:                        pickupDirectory = IisPickupDirectory.GetPickupDirectory();                        break;                    default:                        goto IL_235;                    }                    if (this.EnableSsl)                    {                        throw new SmtpException(SR.GetString("SmtpPickupDirectoryDoesnotSupportSsl"));                    }                    bool allowUnicode = this.IsUnicodeSupported();                    this.ValidateUnicodeRequirement(message, mailAddressCollection, allowUnicode);                    MailWriter mailWriter = this.GetFileMailWriter(pickupDirectory);                    goto IL_275;                    IL_235:                    this.GetConnection();                    allowUnicode = this.IsUnicodeSupported();                    this.ValidateUnicodeRequirement(message, mailAddressCollection, allowUnicode);                    mailWriter = this.transport.SendMail(message.Sender ?? message.From, mailAddressCollection, message.BuildDeliveryStatusNotificationString(), allowUnicode, out ex);                    IL_275:                    this.message = message;                    message.Send(mailWriter, this.DeliveryMethod > SmtpDeliveryMethod.Network, allowUnicode);                    mailWriter.Close();                    this.transport.ReleaseConnection();                    if (this.DeliveryMethod == SmtpDeliveryMethod.Network && ex != null)                    {                        throw ex;                    }                }                catch (Exception ex2)                {                    if (Logging.On)                    {                        Logging.Exception(Logging.Web, this, "Send", ex2);                    }                    if (ex2 is SmtpFailedRecipientException && !((SmtpFailedRecipientException)ex2).fatal)                    {                        throw;                    }                    this.Abort();                    if (this.timedOut)                    {                        throw new SmtpException(SR.GetString("net_timeout"));                    }                    if (ex2 is SecurityException || ex2 is AuthenticationException || ex2 is SmtpException)                    {                        throw;                    }                    throw new SmtpException(SR.GetString("SmtpSendMailFailure"), ex2);                }                finally                {                    this.InCall = false;                    if (this.timer != null)                    {                        this.timer.Dispose();                    }                }            }            finally            {                if (Logging.On)                {                    Logging.Exit(Logging.Web, this, "Send", null);                }            }        }