It is not possible to differentiate between a Director Self Assessment Client and an Individual/Sole Trader Client when sending a bulk email to self assessment only clients as they all use the Self Assessment Service which is the only field you can filter on.
Thanks for the feedback. We appreciate you highlighting this limitation with filtering self-assessment clients. We're actively working on improving this functionality and will have more to share soon.
We added a toggle field, to tick if they are director only.
Most of the times, directors tax return are completed under limited company. The engagement is with LImited company, not with the individual, therefore its hard to remove those directors from the export list of clients.