Sometimes it's known at the moment an address is created that it will need to be configured.
Because the 2nd field is often used from something meaningful other than a number, there are cases where the new address gets hit with more legitimate mail than the 2nd field allows for, and excess messages are deleted. Eg. I created a new account somewhere, and immediately made several changes to that account. Each change was confirmed with an email. So a burst of messages came through and 3 of them were deleted.
I knew it would happen, so I logged into SG, and did a search on the new address, and refreshed the search periodically in hopes that I could get into the configuration page before the messages would hit. I could not time it so that it refreshed after the first message.
The workaround is to send myself a message to that address before disclosing it.. but that doesn't always occur to me, and isn't always convenient - and when I do think of it, sometimes the delay is too great for it to work.