module ActionMailbox::TestHelper

then immediately route it for processing.
{create_inbound_email_from_mail}[rdoc-ref:#create_inbound_email_from_mail],
Create an +InboundEmail+ using the same options or block as

def receive_inbound_email_from_mail(**kwargs, &block)
  create_inbound_email_from_mail(**kwargs, &block).tap(&:route)
end