What are the two main purposes of s/mime?
digital signatures
encryption
junk mail prevention
spam prevention

The two main purpose of S/MIME are digital signatures and encryption

Explanation:

The Secure Multipurpose Internet Mail Extension is used to protect the data that is the data will be encrypted and it is used in security purpose and gives authentication at the receiver and at the sender side

This public key also allows the users to digitally sign the mail and check for the fraudulent in cases thus it allows the digital signing of mails and envelops the data and it is defined in detail in the RFC 3369 and 3370

This has now been also layered in the Cryptographic message syntax