emails are stuck in outbox, which is not displayed as folder in outlook

Unfortunately I was unwise to send a number of emails with large attachments. It seems my email provider cannot handle these, and the emails are stuck in the outbox, and are retrying to send each time. I do not have an Outbox folder visible, where I could delete these errant emails. Can you please tell me how to delete these, even at os-x level. Thank you and regards, Robert