Note that the mere presence of this header causes premailer to be skipped, i.e., even setting skip_premailer: false will cause premailer to be skipped. The reason for that is that the skip_premailer is a simple header and the value is transformed into a string, causing 'false' to become truthy.
They should fix this!
lib/premailer/rails/hook.rb
def skip_premailer_header_present?
message.header[:skip_premailer]
end