bounce = the delivery was attempted but the server responded that it couldn't be delivered. The account may not exist, is full, etc
deferred = the delivery will be delayed by a few minutes to avoid the sending IP from getting on blacklists due to sending too many emails too fast
delivered = delivered
dropped = the email address has requested to not receive this emails, the address is known to bounce, or there is another known reason that the email will not be delivered, so it is not attempted
open = the email was opened
processed = email requests that have been processed but not yet put into one of the other categories by sendgrid.