Transform email header string to header array & replace it at various situations.

src/Object/Email.php

@@ -47,14 +47,14 @@ class Email implements IEmail
 	/** @var string */
 	private $msgText;
 
-	/** @var string */
-	private $additionalMailHeader = '';
+	/** @var string[][] */
+	private $additionalMailHeader;
 	/** @var int|null */
-	private $toUid = null;
+	private $toUid;
 
 	public function __construct(string $fromName, string $fromAddress, string $replyTo, string $toAddress,
 	                            string $subject, string $msgHtml, string $msgText,
-	                            string $additionalMailHeader = '', int $toUid = null)
+	                            array $additionalMailHeader = [], int $toUid = null)
 	{
 		$this->fromName             = $fromName;
 		$this->fromAddress          = $fromAddress;
@@ -127,6 +127,25 @@ class Email implements IEmail
 		return $this->additionalMailHeader;
 	}
 
+	/**
+	 * {@inheritDoc}
+	 */
+	public function getAdditionalMailHeaderString()
+	{
+		$headerString = '';
+
+		foreach ($this->additionalMailHeader as $name => $values) {
+			if (is_array($values)) {
+				foreach ($values as $value) {
+					$headerString .= $name . ': ' . $value . '\n';
+				}
+			} else {
+				$headerString .= $name . ': ' . $values . '\n';
+			}
+		}
+		return $headerString;
+	}
+
 	/**
 	 * {@inheritDoc}
 	 */