When I send an email from CRM, it shows as either Sent, Received or Delivered. Can you explain the difference please?

Sent can show for days or more, so I'm assuming this isn't an email that's in transit. 

Very few emails show as received or delivered, even when the email is correct. 

I doubt it's a case of clients' servers not letting the emails through as other emails to colleagues in the same company will show as opened so I know some are getting thought. 

Please advise.