That only works on mail that actually gets delivered to your mailbox. Gmail can and will: - Reject delivery of mail from sources it has determined to be spammy

That only works on mail that actually gets delivered to your mailbox. Gmail can and will: - Reject delivery of mail from sources it has determined to be spammy (e.g. residential IPs) - this mechanism allows the sender to be aware that delivery failed. - Accept delivery and then silently drop the message somewhere between after accepting delivery but before appearing in the end user's mailbox. In either case, there is nothing you (the mailbox holder) can do. In the second case, there is no way for anyone to know the message hasn't been delivered.