This seems like an error rather than a needed feature. When using the Inquiry(Create) request to POST a new inquiry, email address is only a field for the candidate (often a young child). Not for either of the users created for the household (the parents who actually have email addresses that we need to know).

When parents fill out our live inquiry form, it captures email addresses without issue. The email field is just missing when doing the same via API.

I tried just inserting an "email": field in the request next to wireless_phone or more obscure fields like "business_fax" but it is ignored.