Are you seeing any indications of unknown mail unexpectedly being sent from your account, with copies of unknown messages to unknown recipients still in your sent email folder?
Is this Apple iCloud email, or some other email provider?
At least so far, it is more likely somebody flubbed some web sign-up forms, and others flubbed the sign-up verification process.
Do people flub sign-ups? Yep. I’ve gotten trip-related messages and itineraries and rental receipts for somebody’s Iberian peninsula trip, and endless cheap sneaker ads, among other misrouted email, all because people didn’t know their own email addresses, and the website developers didn’t verify the email addresses submitted.
But at least so far, this looks like a routine email address mistake. Not a hack, and not a virus. Not so far.