@jdaviescoates
you're right, there's the hidden portal settings where support email is, seems things have been shuffled.
I'm not running inbound mailboxes for any of the Ghost installs -- the working ones also have just Mailgun setup for outbound relay and use the regular ghost.app@example.com (with mailgun as smtp relay) and they work.
Yes, I read the setup and have 2 working installs of Ghost.
Yes, I have the IP address whitelisted.
The main difference here is technically I am ghost.example.com, with example.com domain linked to Cloudron, so the emails are attempting to send from example.com.
Is the 550 error from Mailgun? Or from somewhere else?