Older emails sent to other / unprotected servers with the use of a PIN was susceptible to brute-force attacks since we didn't enforce a maximum amount of times the PIN can be guessed by an attacker.
Beginning today, all new emails sent to third party services and protected with a PIN have a limit of 3 wrong entries, and after that the message will be deleted. If a correct PIN is entered within these 3 tries, the counter will be reset. This will protect senders from any possibility that an attacker guesses the 5 digit password connected to their message to a third party service.
There is still danger that if the server gets compromised an attacker can retrieve encrypted data and try to run a successful attack. In the future, we are going to allow our users to pick a custom PIN.