@screnshaw I'm using this and it's working just fine:

email.send({
            author: params.sender,
            recipients: [params.approver],
            subject: message.subject,
            body: message.body,
            attachments: attachments,
            relatedRecords: {
                transactionId: recordId,
                entityId: params.sender
            }
        });

where

recordId

is just

"123"

. The only time you need a

recordType

within

relatedRecords

is when you're attaching to a Custom Record, which would look something like this:

relatedRecords: {
  customRecord: {
    id: "123",
    recordType: "customrecord_blah"
  }
}