Could you please tell when to use Zend_Mail_Transport_Sendmail and when to use Zend_Mail_Transport_Smtp for Zend Mail? What are the benefits of one over another? What is better from the perfomance point of view, for example? If I need to send many emails what is a better choice? I use smpt server on linux server and Green Arrow Delivery System. I used Zend_Mail_Transport_Smtp transport for a while (it seems to me a preferred way, correct me I'm wrong), but for some reason when I use Zend_Mail_Transport_Smtp message id header is not set by server, but it looks like it sets message id header if I use Zend_Mail_Transport_Sendmail. Do you I have some limitations or perfomance (or scalability) penalties if I use Zend_Mail_Transport_Sendmail over Zend_Mail_Transport_Smtp?