How is it possible that mailing systems like Gmail, Yahoo, and Hotmail are able to check the content of an attached password-protected archived file like WINRAR, if the password cannot be guessed, brute-forced, and recovered? Or is it possible that the WINRAR injected a back-door password and selling it to these mailing systems?
Encryption is encryption. Email services won't spend time and energy trying to brute force your password, nor do I believe they would create backdoors for the sake of email. If they let you send encrypted archives then they simply don't check the contents.
I tested. GMAIL blocks all encrypted archives.
I attempted to send a password protected zip containing a txt file, and got the following reply: host gmail-smtp-in.l.google.com[2a00:1450:4010:c05::1a] said: 552-5.7.0 This message was blocked because its content presents a potential 552-5.7.0 security issue. Please visit 552-5.7.0 http://support.google.com/mail/bin/answer.py?answer=6590 to review our 552 5.7.0 message content and attachment content guidelines. w6si5949161lad.65 - gsmtp (in reply to end of DATA command)
So this is a blanket block. It does not matter if the content is safe or not. If they for some reason cannot scan the file, they will regard it as "corrupted" and reject it. Corrupted archives are prohibited. Thus you are protected, but that also means you need other methods for sending confidental data over GMAIL. One good example is to use PGP to encrypt files Before sending. Google wont reject these since they require a manual process to open.
They do not do it.
Attempting to decrypt a message can be illegal depending on the country.
They either send it through as a binary blob or block altogether.