From 874cef6a19fb9fd9f12c11d2477170baa4a7eea2 Mon Sep 17 00:00:00 2001
From: Philipp <admin@philipp.info>
Date: Sat, 19 Sep 2020 20:14:55 +0200
Subject: [PATCH 1/2] Transform email header string to header array & replace
 it at various situations.

---
 phpmailer/phpmailer.php        | 9 ++++++---
 securemail/SecureTestEmail.php | 2 +-
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/phpmailer/phpmailer.php b/phpmailer/phpmailer.php
index e9a3c66ea..2b38f07a6 100644
--- a/phpmailer/phpmailer.php
+++ b/phpmailer/phpmailer.php
@@ -90,9 +90,12 @@ function phpmailer_emailer_send_prepare(App $a, IEmail &$email)
 
 		// additional headers
 		if (!empty($email->getAdditionalMailHeader())) {
-			foreach (explode("\n", trim($email->getAdditionalMailHeader())) as $header_line) {
-				list($name, $value) = explode(':', $header_line, 2);
-				$mailer->addCustomHeader(trim($name), trim($value));
+			foreach ($email->getAdditionalMailHeader() as $name => $values) {
+				// Skip the "Message-ID" header because PHP-Mailer is using its own
+				if ($name == 'Message-Id') {
+					continue;
+				}
+				$mailer->addCustomHeader(trim($name), trim(implode("\n", $values)));
 			}
 		}
 
diff --git a/securemail/SecureTestEmail.php b/securemail/SecureTestEmail.php
index e15ef601f..8341f5451 100644
--- a/securemail/SecureTestEmail.php
+++ b/securemail/SecureTestEmail.php
@@ -54,6 +54,6 @@ class SecureTestEmail extends Email
 
 		parent::__construct($sitename, $sender_email, $sender_email, $a->user['email'],
 			$subject, "<p>{$message}</p>", $message,
-			'', local_user());
+			[], local_user());
 	}
 }

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 2/2] Add improvements

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

diff --git a/phpmailer/phpmailer.php b/phpmailer/phpmailer.php
index 2b38f07a6..5b43c8f19 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)));
 			}
 		}