PeerJ/Swiftmailer-Transport--EsmtpDataReceiptTransport

Adds X-Swift-Data-Receipt to the message header comprised of the smtp data receipt text

Swiftmailer-Transport--EsmtpDataReceiptTransport

Adds X-Swift-Data-Receipt to the message header comprised of the smtp data receipt text

Add to composer.json

"peerj/swiftmailer-transport-esmtpdatareceipttransport": "dev-master"

Add to parameters.yml

parameters: swiftmailer.transport.smtp.class: Swift_Transport_EsmtpDataReceiptTransport

In your implmentation of Swift_Events_SendListener::sendPerformed

$dataReceipt = $message->getHeaders()->get("X-Swift-Data-Receipt")->getValue();