From 5663e61791e996cbfbadff116481f6e20e0639cb Mon Sep 17 00:00:00 2001
From: Philipp <admin@philipp.info>
Date: Sat, 19 Sep 2020 22:49:44 +0200
Subject: [PATCH] Add improvements

---
 phpmailer/phpmailer.php | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/phpmailer/phpmailer.php b/phpmailer/phpmailer.php
index 2b38f07a..5b43c8f1 100644
--- a/phpmailer/phpmailer.php
+++ b/phpmailer/phpmailer.php
@@ -91,11 +91,13 @@ function phpmailer_emailer_send_prepare(App $a, IEmail &$email)
 		// additional headers
 		if (!empty($email->getAdditionalMailHeader())) {
 			foreach ($email->getAdditionalMailHeader() as $name => $values) {
-				// Skip the "Message-ID" header because PHP-Mailer is using its own
+				// Set the "Message-ID" header for PHP-Mailer directly
 				if ($name == 'Message-Id') {
-					continue;
+					// implode all values to one entry, because there's only one value possible
+					$mailer->MessageID = trim(implode("", $values));
+				} else {
+					$mailer->addCustomHeader(trim($name), trim(implode("\n", $values)));
 				}
-				$mailer->addCustomHeader(trim($name), trim(implode("\n", $values)));
 			}
 		}