src/Viserio/Component/Mail/Transport/PostmarkTransport.php
protected function processMessageParts(array $payload, Swift_Mime_SimpleMessage $message): array
    {
        //Get the primary message.
        switch ($message->getContentType()) {
            case 'text/html':
            case 'multipart/mixed':
            case 'multipart/related':
            case 'multipart/alternative':
                $payload['HtmlBody'] = $message->getBody();

                break;

            default:
                $payload['TextBody'] = $message->getBody();

                break;
        }

        // Provide an alternate view from the secondary parts.
        if ($plain = $this->getMIMEPart($message, 'text/plain')) {
            $payload['TextBody'] = $plain->getBody();
        }

        if ($html = $this->getMIMEPart($message, 'text/html')) {
            $payload['HtmlBody'] = $html->getBody();
        }

        if ($message->getChildren()) {
            $payload['Attachments'] = [];

            foreach ($message->getChildren() as $attachment) {
                if (\is_object($attachment) && $attachment instanceof Swift_Mime_Attachment) {
                    $attachments = [
                        'Name'        => $attachment->getFilename(),
                        'Content'     => \base64_encode($attachment->getBody()),
                        'ContentType' => $attachment->getContentType(),
                    ];

                    if ($attachment->getDisposition() !== 'attachment' &&
                        $attachment->getId() !== null
                    ) {
                        $attachments['ContentID'] = 'cid:' . $attachment->getId();
                    }

                    $payload['Attachments'][] = $attachments;
                }
            }
        }

        return $payload;
    }
protected function processMessageParts(array $payload, Swift_Mime_SimpleMessage $message): array
    {
        //Get the primary message.
        switch ($message->getContentType()) {
            case 'text/html':
            case 'multipart/mixed':
            case 'multipart/related':
            case 'multipart/alternative':
                $payload['HtmlBody'] = $message->getBody();

                break;

            default:
                $payload['TextBody'] = $message->getBody();

                break;
        }

        // Provide an alternate view from the secondary parts.
        if ($plain = $this->getMIMEPart($message, 'text/plain')) {
            $payload['TextBody'] = $plain->getBody();
        }

        if ($html = $this->getMIMEPart($message, 'text/html')) {
            $payload['HtmlBody'] = $html->getBody();
        }

        if ($message->getChildren()) {
            $payload['Attachments'] = [];

            foreach ($message->getChildren() as $attachment) {
                if (\is_object($attachment) && $attachment instanceof Swift_Mime_Attachment) {
                    $attachments = [
                        'Name'        => $attachment->getFilename(),
                        'Content'     => \base64_encode($attachment->getBody()),
                        'ContentType' => $attachment->getContentType(),
                    ];

                    if ($attachment->getDisposition() !== 'attachment' &&
                        $attachment->getId() !== null
                    ) {
                        $attachments['ContentID'] = 'cid:' . $attachment->getId();
                    }

                    $payload['Attachments'][] = $attachments;
                }
            }
        }

        return $payload;
    }
Applies the message parts and attachments
into the API Payload.

@param array                     $payload
@param \Swift_Mime_SimpleMessage $message

@return array