Are all the users who received the email registered as Marketo users? If not, and they're in your database as leads, then I would guess something was mis-configured. You can easily find out by searching for the lead's email address in the lead database, and then going through the activity log to see which campaign caused it to be sent out.
Otherwise, I think we'd need more information, like the content, to try and guess what might have happened.
One was a marketo user and the other two were not. They are all in the database as leads. The configuration was done using tokens which has been the same process for over a year. We sent out the campaign and send it to specific emails so people have visibility of what is being sent. This is not a common case then?
Thanks for your answer.