my fix for https://github.com/magento/magento2/issues/24902

m2.3.3.patch

--- vendor/magepal/magento2-gmailsmtpapp/Model/TwoDotThree/Smtp.php
+++ vendor/magepal/magento2-gmailsmtpapp/Model/TwoDotThree/Smtp.php
@@ -78,7 +78,9 @@
         $dataHelper = $this->dataHelper;
         $dataHelper->setStoreId($this->storeModel->getStoreId());

+        $encoding = $message->getEncoding();
         $message = Message::fromString($message->getRawMessage());
+        $message->setEncoding($encoding);

         //Set reply-to path
         switch ($dataHelper->getConfigSetReturnPath()) {

--- vendor/magento/framework/Mail/Template/TransportBuilder.php
+++ vendor/magento/framework/Mail/Template/TransportBuilder.php
@@ -377,6 +377,7 @@ protected function prepareMessage()
     {
         $template = $this->getTemplate();
         $content = $template->processTemplate();
+
         switch ($template->getType()) {
             case TemplateTypesInterface::TYPE_TEXT:
                 $part['type'] = MimeInterface::TYPE_TEXT;
@@ -391,7 +392,10 @@ protected function prepareMessage()
                     new Phrase('Unknown template type')
                 );
         }
+
+        /** @var \Magento\Framework\Mail\MimePartInterface $mimePart */
         $mimePart = $this->mimePartInterfaceFactory->create(['content' => $content]);
+        $this->messageData['encoding'] = $mimePart->getCharset();
         $this->messageData['body'] = $this->mimeMessageInterfaceFactory->create(
             ['parts' => [$mimePart]]
         );
@@ -400,6 +404,7 @@ protected function prepareMessage()
             (string)$template->getSubject(),
             ENT_QUOTES
         );
+
         $this->message = $this->emailMessageInterfaceFactory->create($this->messageData);

         return $this;