One approach to a mail adapter in ParseServer

index.js

// (A user's implementation of Parse Server)
// We can override the default templates with whichever templates we'd like
import { ParseServer, MailgunAdapter, SendGridAdapter } from "parse-server";
import "dust" from "dustjs-linkedin";

// Pretty much every templating language supports compiling templates into functions
const myMailTemplates = {
    "password_reset": {
        "html": require('./views/password_reset_html.js')(dust), // Precompiled dust template
        "text": require('./views/password_reset_text.js')(dust)
    }
};

const server = new ParseServer({
    ...
    mail: {
        templates: myMailTemplates,
        adapter: MailgunAdapter,
        options: {
            apiKey: "mailgun1234",
            defaults: {
                from: "admin@parse.com",
                bcc: ["sales@parse.com"],
            }
        }
    }
    ...
});

// At some later point I can also update the templates
server.mailProvider.getAdapter().setTemplates({ ... });

// Or setup a different mail service
server.setupMailService({
    options: {
        apiKey: "mailgun1234",
        defaults: {
            from: "admin@parse.com",
            bcc: ["sales@parse.com"],
        }
    },
    templates: myMailTemplates,
    adapter: SendGridAdapter
});

// Or just set the adapter manually
server.mailProvider.setAdapter(
    new MailgunAdapter({
        options: {
            apiKey: "mailgun1234",
            defaults: {}
        },
        templates: null // Use the defaults, can also just leave this undefined
    })
);

ParseServer.js

// ParseServer.js
import { default as MailProvider } from 'providers/MailProvider';
import { default as DEFAULT_MAIL_ADAPTER } from 'adapters/MailgunAdapter';

// These default templates live in the parse-server repo
const DEFAULT_MAIL_TEMPLATES = {
    "password_reset": {
        html: jade.compileFile("views/mail/password_reset.html.jade"),
        text: jade.compileFile("views/mail/password_reset.txt.jade")
    },    
    "email_verification": {
        html: jade.compileFile("views/mail/email_verification.html.jade"),
        text: jade.compileFile("views/mail/email_verification.txt.jade")
    }
};

class ParseServer {
    constructor(args) {
        ...
        this.setupMailProvider(args.mail);
        ...
    }
    ...
    setupMailProvider(config) {
        // Merge the default templates with the overrides
        let templates = Object.assign({}, DEFAULT_MAIL_TEMPLATES, config.templates);
        let adapterClass = config.adapter || DEFAULT_MAIL_ADAPTER;
        let adapter = new adapterClass({ options: config.options, templates: templates });
        this.mailProvider = new MailProvider(adapter);
    }
    ...
}

users.js

// Some implementation, i.e. users.js sendPasswordReset
...
// This example assumes we've injected an instance of ParseServer to the request context. 
// We can also continue using the singleton pattern, i.e. new MailProvider() would return the existing instance
// I don't really care, as long as we're explicit about it (not using modules.exports load order to enforce a singleton like we currently are for cache.js);
const mailAdapter = req.Parse.Server.mailProvider.getAdapter();

// Using a precompiled template
mailAdapter.sendMessage({
    to: req.user.get('email'),
    subject: "Reset yo' password",
    templateName: "password_reset",
    locals: { password_reset_link: "https://parse.com/verify?abcd1234" }
});

// Compiling a template a run time
mailAdapter.sendMessage({
    to: req.user.get('email'),
    from: null, // use default
    cc: null, // use default
    bcc: null // use default
    subject: "Reset yo' password",
    template: { html: jade.compileFile("views/password_reset.html.jade") },
    locals: { password_reset_link: "https://parse.com/verify?abcd1234" }
});
...