diff --git a/apps/dav/lib/CalDAV/Schedule/IMipPlugin.php b/apps/dav/lib/CalDAV/Schedule/IMipPlugin.php
index 3ff3ed0c5697a..cd3a9b9391d5c 100644
--- a/apps/dav/lib/CalDAV/Schedule/IMipPlugin.php
+++ b/apps/dav/lib/CalDAV/Schedule/IMipPlugin.php
@@ -244,12 +244,17 @@ public function schedule(Message $iTipMessage) {
 		$template->addFooter();
 		$message->useTemplate($template);
 
-		$attachment = $this->mailer->createAttachment(
-			$iTipMessage->message->serialize(),
-			'event.ics',// TODO(leon): Make file name unique, e.g. add event id
-			'text/calendar; method=' . $iTipMessage->method
+		# We choose to work like Thunderbird Lightning.
+		# using a plain text text/calendar ics.
+		# This plain text text/calendar part is needed for
+		# Microsoft Outlook versions <= 2010 to work.
+
+		$itip_msg = $iTipMessage->message->serialize();
+		$message->addPart(
+				$itip_msg,
+				'text/calendar; method=' . $iTipMessage->method,
+				'UTF-8'
 		);
-		$message->attach($attachment);
 
 		try {
 			$failed = $this->mailer->send($message);
diff --git a/lib/private/Mail/Message.php b/lib/private/Mail/Message.php
index 8a13e2c108cc7..79fc9928930e5 100644
--- a/lib/private/Mail/Message.php
+++ b/lib/private/Mail/Message.php
@@ -58,6 +58,26 @@ public function attach(IAttachment $attachment): IMessage {
 		return $this;
 	}
 
+	/**
+	 * @param $body:  body of the mime part
+	 * @param $content-type = null: Mime Content-Type (e.g. text/plain or text/calendar)
+	 * @param $charset = null: Character Set (e.g. UTF-8)
+	 * @return $this
+	 * @since 16.0.0
+	 */
+	public function addPart($data, $content_type = null, $charset = null): IMessage {
+	       # To be sure this works with iCalendar messages, we encode with 8bit instead of
+	       # quoted-printable encoding. We save the current encoder, replace the current
+	       # encoder with an 8bit encoder and after we've finished, we reset the encoder
+	       # to the previous one.
+               $encoder = $this->swiftMessage->getEncoder();
+               $eightbit_encoder = new \Swift_Mime_ContentEncoder_PlainContentEncoder("8bit");
+               $this->swiftMessage->setEncoder($eightbit_encoder);
+               $this->swiftMessage->addPart($data, $content_type, $charset);
+               $this->swiftMessage->setEncoder($encoder);
+               return $this;
+        }
+
 	/**
 	 * SwiftMailer does currently not work with IDN domains, this function therefore converts the domains
 	 * FIXME: Remove this once SwiftMailer supports IDN
diff --git a/lib/public/Mail/IMessage.php b/lib/public/Mail/IMessage.php
index 638fd9d103fc7..b4073bee535ef 100644
--- a/lib/public/Mail/IMessage.php
+++ b/lib/public/Mail/IMessage.php
@@ -39,6 +39,15 @@ interface IMessage {
 	 */
 	public function attach(IAttachment $attachment): IMessage;
 
+	/**
+	 * @param $body:  body of the mime part
+	 * @param $content-type = null: Mime Content-Type (e.g. text/plain or text/calendar)
+	 * @param $charset = null: Character Set (e.g. UTF-8)
+	 * @return IMessage
+	 * @since 16.0.0
+	 */
+	public function addPart($body, $content_type = null, $charset = null): IMessage;
+
 	/**
 	 * Set the from address of this message.
 	 *