During the pandemic, I launched [link redacted], an open-source tool to check if an email exists, without sending any email. Not sure what "successful" means here, but I have a couple dozens of customers, which for me is a success.
Many mail servers function by answering all “RCPT TO:” commands (for the domains served) with a success message that is effectively “Yeah, sure... give it a try” and, if the mailbox doesn’t exist, will email the sender a bounce message.