Reply-to header not being sent correctly – FIX

Similar to issues reported regarding BCC and CC fields, emails sent via WP SendGrid don’t get the reply-to header attached to them correctly. It would be great to have this worked directly into the plugin, but I was able to fix the problem using the following filter. Posting it here in case others find it useful.

add_filter( 'wp_sendgrid_args', 'my_append_reply_to_filter', 10, 6 );
function my_append_reply_to_filter( $args, $to, $subject, $message, $headers, $attachment_array ) {
  // The WP SendGrid plugin doesn't send the replyto argument to SendGrid
  // like it should, so we have to append it
  // SendGrid API: https://sendgrid.com/docs/API_Reference/Web_API/mail.html
  // $args is sent to SendGrid in the request

  if ( !empty( $args[ 'replyto' ] ) )
    return $args;

  $reply_to = !empty( $headers[ 'reply-to' ] ) ? $headers[ 'reply-to' ] : '';
  $reply_to = !$reply_to && !empty( $headers[ 'Reply-To' ] ) ? $headers[ 'Reply-To' ] : $reply_to;
  $reply_to = !$reply_to && !empty( $headers[ 'REPLY-TO' ] ) ? $headers[ 'REPLY-TO' ] : $reply_to;

  if ( $reply_to )
    $args[ 'replyto' ] = $reply_to;

  return $args;

}

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