split mail and mailserver

mail = all the per-domain code
mailserver = all the mail server level code

src/mailer.js

@@ -20,7 +20,7 @@ const assert = require('assert'),
     branding = require('./branding.js'),
     debug = require('debug')('box:mailer'),
     ejs = require('ejs'),
-    mail = require('./mail.js'),
+    mailServer = require('./mailserver.js'),
     nodemailer = require('nodemailer'),
     path = require('path'),
     safe = require('safetydance'),
@@ -51,7 +51,7 @@ async function sendMail(mailOptions) {
         return;
     }
 
-    const data = await mail.getMailAuth();
+    const data = await mailServer.getMailAuth();
 
     const transport = nodemailer.createTransport({
         host: data.ip,