MailMessage message = new MailMessage();
message.From = new MailAddress("from@server.com", "From Name");
message.To.Add(new MailAddress("to@server.com", "To Name"));
message.Subject = "Email with attachment";

byte[] data = new UTF8Encoding().GetBytes("Content of attachment");
MemoryStream ms = new MemoryStream(data);

//Adds an attachment from the Stream
message.Attachments.Add(new Attachment(ms, "file.txt", "text/plain"));

SmtpClient emailClient = new SmtpClient("mail.server.com");
emailClient.Send(message);