Email Delivery Status

Hi, I just wanted to ask about email delivery status for both Candidates and Contacts. I can see there is: 'sent', 'delivered' 'opened'.
I am trying to understand if these status codes are accurate - we have sent a number of emails where they are sitting at 'sent' for over 24-hours - does this mean the email is stuck in the server and the client contact or candidate has not seen them arrive? Also, if the email is shoiwng as 'delivered' but not 'opened' can we be confident the email has not been read? Many thanks