Good question! That one is covered in the README 
If you don’t use Hanami View, override the internal rendering methods. These are also a hook for integrations with other rendering systems.
class CustomMailer < Hanami::Mailer
from "custom@example.com"
to { |user:| user[:email] }
subject "Custom Email"
expose :user
private
def render_view(format, input)
user = input[:user]
case format
when :html
<<~HTML
<html>
<body>
<h1>Hello, #{user[:name]}!</h1>
</body>
</html>
HTML
when :text
"Hello, #{user[:name]}!"
end
end
end
The dependency on hanami-view is completely optional. If it’s available (i.e. in your bundle), then mailer rendering will use it. You can also disable it explicitly with config.integrate_view = false.