@kescher I would personally probably reply explaining that it is a confirmation e-mail for a signup and that no further e-mails will be sent to them unless they confirm their account - I would expect the notification to be automatic, but the reply (if any) to be handled manually.
*However*, I don't know what the current e-mail landscape looks like, so this is definitely *not* an answer from an 'experienced mail admin' perspective, and it's possible there's something I'm not aware of.