How should I use Multi threaded emails using codeigniter

Just wanted to implement sending emails and fetching replies in a single thread using Codeigniter ...

Code:

$this->email->set_header('CUSTOM-ID', '#1234567');

I have tried to pass a CUSTOM-ID , say #1234567 for the mail I sent from my application ,  how should  I receive this  CUSTOM-ID back in my replies  from the mail server (eg - Gmail) ?

Or , Is there any other better way to implement the threaded email functionality using CodeIgniter . Please advise . 

Comments and suggestions are welcome  . 

Thank You 

have you tried pthreads? native multi-threading in php

God Bless CI Contributors