Email Send Sometimes and sometimes Not

Question

I am using my domain account address [email protected] to send email to gmail,Yahoo and other domain account addresses , It send correctly  with gmail,yahoo mail  but with some specific domain like [email protected] it send emails randomly sometimes it send and some times it did not send , I have method on the button click whcih  call another method and at the end this method call the email send method. below is my method which works fine for gmail and yahoo accounts but did not work for the specific domain account  like [email protected], why is it so i am checking that from past two days but i could not  find a solution  yet .

 

 

Thanks in advance