derive child keys from a private key
If key revocation is a requirement, that is, if Alice no longer works for BigCorp, her keys no longer is able to decrypt email to BigCorp, then it either becomes impractical or easy.

That is, you will need to do one of two things:


Either you need to update the master public key when Alice leaves.  This is likely to be impractical.
You require Alice to work with some central body to do the decryption; the central body would verify Alice's employment as a part of the decryption process.


In the latter case, where we can assume a trusted central body, it becomes easy; have the central body hold the private key; it would either decrypt the message for Alice, or alternatively (for example, with RSA or IES), the central body could do a blinded decryption (and so the entity won't be able to track the message)