If your client does not have an email address, CMS recommends helping them create one, so the client can have access to their application if they need it.
However, if they do not wish to create an email address, ACA agents can skip this field when writing an application through their HealthSherpa for Agents account.
Please keep in mind that if you don't list an email for a client:
They'll still get paper mail from the Marketplace
They won't be able to use their Consumer Dashboard through HealthSherpa, which allows them to do their own servicing year-round.