The email sent from the Smart Key app might take up to 10 minutes to arrive.
Make sure you entered the email address correctly. To verify, go to the sharing page of your key and see the list of users with whom you shared this key. You can try to send a simple message to this email address as well, just to verify whether the email is correct.
Often emails wind up in Spam folders or are caught by Spam blockers. So, if you have shared a key but the recipient has not received an email, first ask the recipient to check their Spam folder. In case of a Gmail account, the recipient may have to check in their “Promotion” folder. In the case of a Spam message, the attachment might be invisible.