mail-tester.com only performs very simple tests, mostly SPF/DKIM… Anyone running Mail in a Box on a non-blacklisted IP should get 100%.
Like aspdye says, it’s in the admin area. Leave Subdomain blank, set Type to TXT, and paste the string Google gives you for Value. Takes all of 20 seconds.
Good suggestion, hoping this helps with gmail… Most of my mail was getting marked spam too.