ErrorCode 403 : Invalid request field(s): $.Metadata

[tflight]

Hi,

I've been getting this error when sending some emails. The emails which it fails on seem fairly random, though it occurs more commonly with some types of emails than others.

{"ErrorCode":403,"Message":"Invalid request field(s): $.Metadata"}

I've read this article on Changes to Email API validation and it seems related. I am using the postmark_metadata filter in my emails. So I'm unsure if I'm doing something incorrect or if the plugin should be updated to do additional validation before sending along the email.

postmark-wordpress/wp-mail.php

Lines 234 to 237 in aa523b7

	// Support using a filter to set metadata on a message
	if ( has_filter( 'postmark_metadata' ) ) {
	$body['Metadata'] = apply_filters( 'postmark_metadata', array() );
	}

I'm using the filter like this:

add_filter('postmark_metadata', 'cc_postmark_metadata');
function cc_postmark_metadata($metadata)
{
    if (isset($_SERVER['REMOTE_ADDR'])) {
        $metadata['remote_ip'] = $_SERVER['REMOTE_ADDR'];
    }

    return $metadata;
}

I've turned off this filter for now to make sure this is where the issue is coming from.