Username: noreply@domain.com
Password: password123
Incoming Server: mail.domain.com (IMAP Port: 993, POP3 Port: 995)
Outgoing Server: mail.domain.com (SMTP Port: 465)

MailDefinition md = new MailDefinition();
md.From = "noreply@domain.com";
md.IsBodyHtml = true;
md.Subject = "subject";

MailMessage mm = md.CreateMailMessage("user1@gmail.com", null, "Some Text", new System.Web.UI.Control());
mm.From = new MailAddress("noreply@domain.com", "System Name");

SmtpClient smtp = new SmtpClient();
smtp.UseDefaultCredentials = false;
smtp.Credentials = new NetworkCredential("noreply@domain.com", "password123");
smtp.Host = "mail.domain.com";
smtp.Port = 465; //tried for 25, 587
smtp.EnableSsl = true;
smtp.Timeout = 40000;
smtp.Send(mm);

<system.net>
    <mailSettings>
      <smtp deliveryMethod="Network" from="noreply@domain.com">
          <network host="mail.domain.com" port="465" userName="noreply@domain.com" password="password123" defaultCredentials="false" enableSsl="true" />
      </smtp>
    </mailSettings>
</system.net>

Port 25: Mailbox unavailable. The server response was: Error458 block
Port 26: Mailbox unavailable. The server response was: Error458 block
Port 465: The operation has timed out
Port 587: Mailbox unavailable. The server response was: Error458 block