SPUtility.SendEmail() always returns false

I have one application which will create task form for each level based on the prior level approval. While creating each tasks I have to send the email to the users who all are involved in the task. For this I am using SPUtility.SendEmail() method. But unfortunately, it didn’t work so far. The debugger successfully passed through the SendEmail method without any exception. but the value will always false. The SMTP server is working for the application other than the SP Applications. My code is shown below.

  SPSecurity.RunWithElevatedPrivileges(delegate()             {                 using (SPSite spSite = new SPSite(SiteURL))                 {                     using (SPWeb spWeb = spSite.OpenWeb())                     {                         headers = new StringDictionary();                         headers.Add("to", To);                         headers.Add("from", From);                         headers.Add("cc", CC);                         headers.Add("bcc", BCC);                         if (Priority.Equals("High"))                         {                             headers.Add("X-Priority", "1 (Highest)");                             headers.Add("X-MSMail-Priority", "High");                             headers.Add("Importance", "High");                         }                         headers.Add("subject", Subject);                         headers.Add("content-type", "text/html");                         Status= SPUtility.SendEmail(spWeb, headers, Body);                     }                 }             }); 

Please help me to resolve this issue. All Suggestions would be appreciated. Thanks in advance.