user validation error with mailgun connector loopback: email exists - node.js

Question

I took the example project here and added the loopback-connector-mailgun. I configured it like this:
In datasources.json added the following:
{
"mailgun": {
"connector": "loopback-connector-mailgun",
"apikey": "[my api key here]",
"domain": "[my domain here]"
}
}
then in model-config.json:
{
"Email": {
"dataSource": "mailgun",
"public": false
}
}
When I register a client by doing a post in localhost:3000/api/clients the verification email is sent successfully but the server does not redirect me to the home page and after some time (it hangs for around 30 seconds) I get the following error:
{"name":"ValidationError","status":422,"message":"The `client` instance is not valid. Details: `email` Email already exists (value: \"a#b.com\") ...
The full stack is reported in this gist.
My client.js function:
Client.afterRemote('create', function(context, client, next) {
console.log('> client.afterRemote triggered');
//console.log(typeof client);
//var auth = dsConfig.emailDs.transports[0].auth;
var options = {
type: 'email',
to: client.email,
from: "noreply#loopback.com",
subject: 'Thanks for registering.',
template: path.resolve(__dirname, '../../server/views/verify.handlebars'),
redirect: '/verified',
user: client
};
client.verify(options, function(err, response) {
console.log(err + 'Debug point 1');
if (err) return next(err);
console.log('> verification email sent:', response);
context.res.render('response', {
title: 'Signed up successfully',
content: 'Please check your email and click on the verification link ' +
'before logging in.',
redirectTo: '/',
redirectToLinkText: 'Log in'
});
});
});
The execution never reaches Debug Point 1. Thanks!