koa-help.js

app = koa();

app.use(route.post('/sendmail', sendMail));

function *sendMail(email) {
  var body = yield parse.form(this);
  signup.save(body.email, function (res) {
    this.body = res;
  });
}

from #koajs

I am still learning myself,so take it with a grain of salt. Your signup.save needs to return a promise. You can change it, or use Q.denodeify. Then it gets easy:

function *sendMail(email) {
  var body = yield parse.form(this);
  var response = yield signup.save(body.email);
  this.body = response;
}