Hey Stephen,
Have you checked your spam folder?
I would also make sure that your lead is not subscribed and that you are not black listed. Then send yourself an email via a Smart Campaign to see if you receive the email.
If you do not receive the email, check your activity log for the bounce message Marketo received and that will provide more information into why you are not receiving emails from Marketo. If there is an "Email Delivered" activity but you are still not receiving the emails, I would consult your IT team to see why they emails are not reaching your inbox even though the email server is sending Marketo a Delivered message. I would also recommended to whitelist Marketo IP addresses as descrbied here: https://community.marketo.com/MarketoArticle?id=kA050000000KypECAS
White listing is a great way to increase deliverability to your own email server.
Best,
Will