Subaccounts are an additional way for users who send on behalf of many others, or who operate multi-tenant systems to separate sending for different users (besides using tags or metadata, for example).
With subaccounts, you can separate reputation, activity, reports, and quotas for different types of senders in a single Transactional Email account.
You'll need to add SPF and DKIM records and verify ownership of your sending domains before you can send email through your account. Transactional Email will not send any email from unverified domains or domains without valid SPF and DKIM records, including public domains like gmail.com, yahoo.com, and more.
A message that is rejected with the reject reason unsigned indicates that the sending domain hasn't been properly set up, and that your account is unable to send and authenticate email from that domain.