Email cannot be send to non members in SharePoint 2013

Your question isn't complete as you have not specified what type of SharePoint Application you are building.

In SharePoint 2013, you have several flavours of application development such as Full Trust Solution, SharePoint App etc.

Anyway, let's assume you have a Full Trust Solution. The default SPUtility SendMail functionality in SharePoint is limited. And you can only send emails to people who are known within the site collection. If you only want to send email to people within site collection, then make sure you use EnsureUser() to check the availibility in site collection.

Something like this:

private void SendEmail(ClientContext clientContext)
{
	User sendToUser = clientContext.Web.EnsureUser("abc@def.com");
	clientContext.Load(sendToUser);
	clientContext.ExecuteQuery();

	string email = Microsoft.SharePoint.Client.Utilities.Utility.GetCurrentUserEmailAddresses(clientContext).Value;

	Microsoft.SharePoint.Client.Utilities.EmailProperties properties = new Microsoft.SharePoint.Client.Utilities.EmailProperties();
	properties.To = new string[] { sendToUser.Email };
	properties.Subject = "subject";
	properties.Body = "body";

	Microsoft.SharePoint.Client.Utilities.Utility.SendEmail(clientContext, properties);

	clientContext.ExecuteQuery();
}

Here is an article which can give you a head start (and code) on how to do this. (Full Disclaimer: This refers to my own post on my blog).

http://manasbhardwaj.net/how-to-send-email-using-sharepoint/