It's recently struck me that there is an important difference betweentypes (and classes) created in a Python module and types/classes thatcome from a C-level extension module.

Suppose that duck typing is not enough and soyou really want to make a class that inherits from an outside class(one in another module), yet overrides all of its behavior. This letsyou create objects that work the way you need them to but will pass isinstance() checks that are insisting on instances of the

Normally, Exim routers will only accept or fail addresses (or beuninterested in them). This is good enough for normal handlingof addresses, but if you are using routers to their full power , there are times when you want to force routers todefer addresses instead. There are two general ways to do this.

(Unsuccessful DNS lookups can cause addresses to defer, but thisis not normally under your control.)

The straightforward way is to use a separate router to explicitly deferthe address using the :defer

Officially, as reflected in the documentation, Exim routers are expected to take more or less disjoint sets ofaddresses; for example, you have one router to do DNS lookups and SMTPfor external addresses, one router to handle aliases, one router toexpand the .forwards of people with them, and one router to deliver topeople's mailboxes for people without .forwards. This makes the orderingof the routers relatively unimportant; approached this way, it is usedmostly to make writing routers more convenient by having to