با سلام و احترام

ضمن تشکر از زحمات شما،نمونه کد زیر تست شده و مشکلی نداره که میتونید در صورت تمایل استفاده نمایید.

منصور عزیزی

            SmtpClient mySmtpClient = new SmtpClient("webmail.yourdomain.ir", 25);
            mySmtpClient.Credentials = new System.Net.NetworkCredential()
            {
                UserName = "info@yourdomain.ir",
                Password = "12583214A@rf"
            };
            mySmtpClient.EnableSsl = false;

            // add from,to mailaddresses
            MailAddress from = new MailAddress("info@yourdomain.ir.ir",your Co");
            MailAddress to = new MailAddress(Reciver.ExternalReciver.Email);
            MailMessage myMail = new System.Net.Mail.MailMessage(from, to);

            // add ReplyTo
            MailAddress replyto = new MailAddress("info@yourdomain.ir");
            myMail.ReplyToList.Add(replyto);

            // set subject and encoding
            myMail.Subject = "ECE Email";
            myMail.SubjectEncoding = System.Text.Encoding.UTF8;

            // set body-message and encoding
            myMail.Body = "<b>Test Mail</b><br>using <b>HTML</b>.";
            myMail.BodyEncoding = System.Text.Encoding.UTF8;
            // text or html
            myMail.IsBodyHtml = true;

            if (attachmentFilename != null)
            {
                Attachment attachment = new Attachment(attachmentFilename, MediaTypeNames.Application.Octet);
                ContentDisposition disposition = attachment.ContentDisposition;
                disposition.CreationDate = File.GetCreationTime(attachmentFilename);
                disposition.ModificationDate = File.GetLastWriteTime(attachmentFilename);
                disposition.ReadDate = File.GetLastAccessTime(attachmentFilename);
                disposition.FileName = Path.GetFileName(attachmentFilename);
                disposition.Size = new FileInfo(attachmentFilename).Length;
                disposition.DispositionType = DispositionTypeNames.Attachment;
                message.Attachments.Add(attachment);
            }
            try
            {
                mySmtpClient.Send(myMail);
                return true;
            }
            catch (Exception ex)
            {
                return false;
            }