You won’t like my answer.
There’s just no way to know.
At least, not unless someone did get it and then tells you they did.
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Email to an invalid address
Different email systems handle things differently. This means many different things could happen to your misdirected email.
- You may get a bounce telling you the message was undeliverable. This is probably the best possible situation, as you’d know it wasn’t delivered to the mailbox you sent it too.
- It may be silently discarded. No human may ever see it, and you wouldn’t get notification one way or another.
- It may have been forwarded to a “catch-all” mail address. Many email servers have the ability to forward email sent to an invalid address to one central address. The idea is someone would decide if it was really meant for someone else and forward it. Thanks to spam, this technique is extremely rare these days, but it could happen.
- The bad address may be valid. I know, you said you’re 100% sure — but how can you really be sure? You could be surprised. And clearly this is a case where you don’t want to be surprised.
Remember also that system administrators can see all mail, valid or not. Whether they do is unknown, but they could.
The bottom line: someone might see it. Or not. There’s just no way to know.
What to do?
The reality is, once you hit Send, you lose all control of email. There is nothing you can do to recall, un-send, or otherwise prevent whatever happens next from happening.
What you choose to do next depends on the situation and your message. Perhaps you need to explain to the unintended recipient. Perhaps you need to apologize. Perhaps you need to start looking for a new job.
More likely, you need do nothing until you hear that the message was in fact seen by someone.