As well as creating outbound emails in the UI you can also send them with the sendNewEmail API. This is useful for proactively reaching out about issues. 
import { PlainClient } from '@team-plain/typescript-sdk';

const client = new PlainClient({ apiKey: 'plainApiKey_xxx' });

const res = await client.sendNewEmail({
  customerId: 'c_01GTC6ZHCMAGR06FMPN9VY5J95',
  threadId: 'th_01HD44FHMCDSSWE38N14FSYV6K',
  subject: 'Your account has a problem',
  textContent: 'The plain text version of your email goes here.',
  markdownContent: 'The markdown **version** of your _email_ goes here.',
});

if (res.error) {
  console.error(res.error);
} else {
  console.log(res.data);
}