Section 3.4.3.2 of RFC 3851 says:
The SHA-256, SHA-384, and SHA-512 algorithms [FIPS180-2] are not
currently recommended in S/MIME, and are included here for
completeness.
I think it is time to revisit this decision. I think the standard needs to
accept longer hash functions, at least with the RSA signature algorithm.
Russ