Stumbled on something interesting - checking a class to see if it "is a" or is "kind of" a parent class didn't work. Checking using the inheritence operator did work, as well as looking at the list of ancestors.

irb(main)> class MyMailer < ActionMailer::Base ; end
=> nil
irb(main)> MyMailer.is_a? Class
=> true
irb(main)> MyMailer.kind_of? Class
=> true
irb(main)> MyMailer.is_a? ActionMailer::Base
=> false
irb(main)> MyMailer.kind_of? ActionMailer::Base
=> false

irb(main)> a = MyMailer.new
=> #<MyMailer:0x007fa6d4ce9938>

irb(main)> a.is_a? ActionMailer::Base
=> true
irb(main)> a.kind_of? ActionMailer::Base
=> true

irb(main)> !!(MyMailer < ActionMailer::Base)
=> true
irb(main)> !!(MyMailer < ActiveRecord::Base)
=> false
irb(main)> MyMailer.ancestors.include? ActionMailer::Base
=> true

I suppose .is_a? and .kind_of? are designed as instance-level methods on Object. Classes inherit from Module, which inherits from Object, so a class is technically an instance. These methods will look at what the class is an instance of - pretty much always Class - and then check the ancestors of that.

tl;dr when trying to find out what kind of thing a class is, use the inheritence operator or look at the array of ancestors. Don't use the methods designed for checking the type of an instance of something.