Mechanism of Email encryption by cert

Question

i read article and concludes below, but i still confused about that
A. a sender with cert can send digital signature to receiver, and receiver can encrypt mail with sender's public key, than sender can decrypt by privte key.
B. digiXXXX give me some explianatiuon but i doubt about them,
" the recipient does not needs a cert to decrypt the mail, the key is send with the email by the cert, and so "the recipient uses private key to decrypt the email" is what happens in the backend when the email was received and opend" - from DigiXXX

1. i doubt about B. only sender has bought cert, recevier doesnt, how recipient can has sender's private key?
2. if only sender bought a cert, i want to clarify
   sender can send encrypted email to receiver?
   receiver can send encrypted email to sender?
3. when i use MSOutlook to receive a encrypted email, i cannot read it even added sender's public key by signature in Outlook contact. i guess it is public key so can not be used for decryption, (i recevier without cert, sender has cert)
4. i try to send a encrypted email to sender by his public key, Outlook ask me a cert for encryption......... i doubt both side should have cert also for sending encrypted mail??? DigiXXXX tell me only sender has cert should be able to send encrypted mail.....

thanks.

Answer 1

the recipient does not needs a cert to decrypt the mail, the key is send with the email by the cert

this statement is not correct. Recipient MUST have private key to decrypt the email. Recipient do not need the cert to validate the signature, because they key is indeed sent along with email. In other words, statement A is correct, statement B is not.

sender can send encrypted email to receiver?

no, because senders needs to have recipient's public certificate in order to encrypt it for selected recipient.

receiver can send encrypted email to sender?

as long as recipient has a public certificate of the original sender. Certificate exchange often occurs by sending signed emails. For example, sender sends signed email to recipient and then recipient can take a public certificate from signed email and encrypt response to original sender.