Function send_mail 

pub fn send_mail(
    workspace: &Path,
    from_session: &str,
    mail: &NewMail,
) -> Result<MailReceipt, SessionStoreError>

Send a validated message to a live session’s inbox.

Fails closed when the target session is unknown or not live, when its inbox is full, or when the envelope exceeds MAX_MAIL_BYTES.