Provide an alter hook before sending the email

This patch is OK.

Note that, as specified in the issue properties, it does not apply to the current release, but only to the DEV version. But the parts of the patch that does not apply are related to the README.txt file, so the patch can be used on the current release for those who need it.

Applied patch to 7.x-1.x-dev. Thanks!

Here is my port for D8. I have added third parameter to the alter hook : $message which holds the original drupal message array, which may be useful to pass additional data to the hook.

fixing patch carriage return.

+++ b/src/Plugin/Mail/SwiftMailer.php
@@ -358,6 +358,9 @@ public function mail(array $message) {
+      \Drupal::moduleHandler()->invokeAll('swiftmailer_alter', [&$mailer, &$m, $message]);

We should inject the module handler.

Fix #10. I've changed the comment a little and remove the reference to the objects, which usually are not necessary and to keep it on par with D7.

Patch no longer applies.

+++ b/src/Plugin/Mail/SwiftMailer.php
@@ -353,6 +353,9 @@ class SwiftMailer implements MailInterface, ContainerFactoryPluginInterface {
+      $this->moduleHandler->invokeAll('swiftmailer_alter', [$mailer, $m, $message]);

Isn't it possible to user \Drupal\Core\Extension\ModuleHandlerInterface::alter?

+++ b/src/Plugin/Mail/SwiftMailer.php
@@ -61,6 +62,11 @@ class SwiftMailer implements MailInterface, ContainerFactoryPluginInterface {
+  protected $mailer;

unused property.

1. +++ b/src/Plugin/Mail/SwiftMailer.php
   @@ -384,9 +385,17 @@ class SwiftMailer implements MailInterface, ContainerFactoryPluginInterface {
   +            $mailer = Swift_Mailer::newInstance($transport);
   

   This happens in all cases, so I think it can just be moved to after the switch.

2. +++ b/swiftmailer.api.php
   @@ -0,0 +1,31 @@
   +  $swiftMessage->setReadReceiptTo('your@address.com');
   

   The $addresses parameter is type-hinted as an array, so maybe put the address in an array.

Looks good to me, thanks.