Formal way to tell someone they accidentally sent you someone else’s email?

As you mention that the sender is your "senior", you want to tread carefully.

You could simply state:

I believe this email was sent to me by mistake and wanted to make you aware of it possibly reaching the wrong destination. If this message was intended for me, I look forward to discussing the matter with you further.

Or, if doing so would not "step on toes", you could simply stop by their desk/office and tell them in person.

Don't call it a mistake, and start by mentioning its a common error.

I think you meant to send this to someone else. Just letting you know so you can send it on to the right recipients (or correct me if I misunderstood). Have a great day.

Try:

I'm afraid I wasn't meant to receive this. Thank you.

I would probably say something like:

I'm afraid I don't quite understand. Is it possible that this message was intended for someone else?

Which implies that, if it wasn't a mistake for you to have received the message, you at the very least don't know what to do with it.