Undelivered mail returned to sender

I only use icloud mail to receive receipts from Apple purchases, but every time I check in with icloud mail, I have several "Undelivered mail returned to sender" emails. Something in my Apple mail system is trying to send emails to a Mindspring account I haven't had for 12 or 13 years, at least. I can't track it down. I don't know what's generating the email, and there's no message content to give me a clue. Any ideas?