The specified string is not in the form required for an e-mail address

using (MailMessage message = new MailMessage())
            {
                MailAddress from = new MailAddress("Sender@domain.com", "Customer Information");
                message.From = from;

                to = ChangeSeprators(to); //to split input string. e.g. abc@domain.com;def@domain.com
                if (!string.IsNullOrEmpty(to))
                {
                    message.To.Add(to);
                }

                cc = ChangeSeprators(cc);//to split input string. e.g. abc@domain.com;def@domain.com
                if(!string.IsNullOrEmpty(cc))
                message.CC.Add(cc);
                
                message.Subject = emailSubject;
                message.Body = emailBody;
                message.Attachments.Add(new Attachment(attachment));
                message.BodyEncoding = Encoding.UTF8;
                message.IsBodyHtml = true;

                SmtpClient smtpClient = new SmtpClient(server);
                try
                {
                    smtpClient.Send(message);
                }
                catch (Exception ex)
                {
                    throw new Exception(ex.Message);
                }
            }