The Send Email activity that is logged is based solely on the email being sent by the Marketo email server. After a Send Email activity is logged, a Email Delivered or Email Bounced activity will be logged as well, but this could occur hours later. It is up to the recipient's email server to send a reply back to Marketo's email server and depending on how the recipient email server is configured and its load, it may take several hours to get the response back.