I have utility class to send email.
currently i am facing error :-
a recipient must be specified
EmailProperties emailp = new EmailProperties(); emailp.To = new string[] { "[email protected]" }; emailp.From = email; emailp.Body = "<b>Hello</b>"; emailp.Subject = "New Email"; Utility.SendEmail(clientContext, emailp); clientContext.ExecuteQuery(); 
@Pradip already mentioned that it is not possible to send email to the external people using Utility so can consider System.Net.Mail to serve your purpose.
MailAddress from = new MailAddress("[email protected]", "Jane " + (char) 0xD8 + " Clayton", System.Text.Encoding.UTF8); MailAddress to = new MailAddress("[email protected]"); MailMessage message = new MailMessage(from, to); Find full reference in MSDN
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.
Snippet:
private void SendEmail(ClientContext clientContext) { User sendToUser = clientContext.Web.EnsureUser("[email protected]"); 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(); } Refer this good article by Manas Bhardwaj for more.
clientContext.