If registered mail is sufficient and that only requires proof of delivery/receipt, why would the same thing for email be insufficient?
Email with proof of receipt (e.g. Outlook's read receipts) would be fine. But sending an email and hoping for the best doesn't meet that condition.
Email with proof of receipt (e.g. Outlook's read receipts) would be fine. But sending an email and hoping for the best doesn't meet that condition.