Sends an email to the customer from the project's "Send emails from" address (Project → Emails; 422 if it is not set),
with the reply signature appended and {placeholders} ({email}, USER_ID, {project_name} …) filled in.
The conversation becomes open if it was unread or closed; an unassigned conversation is assigned to the sender (client keys).

Send multipart/form-data with attachments[] to attach files (up to 10, 20 MB each). To make a retry safe, pass a
client_key (or an Idempotency-Key header): the same key within 15 minutes returns the first message instead of emailing again.
For the demo conversation the reply is stored but nothing is sent.