Hi,
You're honestly running into much trouble.
Whenever you get changes into the nginx config pushed from Grommunio, your setup may stop working.
From a security point of view, this is disastrous.
Wordpress is the most attacked / hacked CMS in existence.
Putting it on the same server as your mails is playing with fire.
There are plenty of small server instances for less than a couple of €/$ for this kind of hosting.