https://www.securemyemail.com offers a "$99.99 Limited-Time Lifetime plans". No idea how good they are but it has a 30 day free trial. No guarantee the lifetime subscription offer will still be available 30 days from now, it seems to be a periodic sale they offer to a limited number of customers.
They are a product of Witopia, which was founded in 2005.
https://www.pcmag.com/reviews/securemyemail
https://www.securemyemail.com/encryp...ail-downloads/