But spam is mostly from GMail nowadays. In the last seven days, 8 out of 10 spam mails I received (that didn’t get rejected outright) came from GMail.
When this happens, gmail informs the sender that the mail wasn’t delivered and they try a few more times before telling the user that no more attempts will be made.
That depends entirely on how the receiving server rejects the mail. If it just drops it into the spam folder or /dev/null Gmail won't be able to know. If it responds with a non-transient error code then Gmail will (correctly) NOT try again and inform the user of that.