Ghost/ghost/admin/app/helpers/full-email-address.js

import Helper from '@ember/component/helper';
import {inject as service} from '@ember/service';

export default class FullEmailAddressHelper extends Helper {
    @service config;

    compute([email]) {
        if (email.indexOf('@') > -1) {
            return email;
        }

        return `${email}@${this.config.emailDomain}`;
    }
}