We will call this dynamic stack routing. Theoretically, dynamic stack routing should be transparent to the mail routing application, and is thus not a part of dynamic mail routing. It is mentioned here because in existing products, dynamic stack routing is often very well visible at the mail configuration level, so MTA managers should at least be aware of it. Although static routing is often table based, not all table based routing algorithms are necessarily static in nature. For reasons of flexibility and reliability, dynamic routing is almost always the preferred method.
Mail routing in most existing networks is upto some degree indirect.
Note that local knowledge should be used to route messages within the address space behind the firewall. An example for this is presented later on. 1711 Classifications in E-mail Routing October 1994 exist many reasons for using firewalls, e.g., security considerations or to concentrate the management for a given domain onto one well managed system. The Internet mail system would allow all mail hosts connected to the Internet to directly accept mail from any other host, but not all hosts use this possibility. Apart from that, the need to use explicit source routing has disappeared over the last decennia.