Object address not found on party or party is marked as non-emailable

I am using CRM 2011 & getting below error when trying to send an email . 

"Object address not found on party or party is marked as non-emailable"

I looks into this error & found that its because user does not have valid email address to send an email . 

Can anybody help me how to solve this ?

Below is My Code :

Entity Email = new Entity("email");
Email.Attributes["from"] = from.ToArray();
Email.Attributes["to"] = to.ToArray();
Email.Attributes["subject"] = subject;
Email["description"] = MessageBody.ToString();

Guid EmailId = service.Create(Email);
SendEmailRequest req = new SendEmailRequest();