Backbone basics
The term "email backbone" means that mail originating in a local email system travels from that point onto the email routing backbone, typically through a gateway or MTA (Message Transfer Agent), and is transported through the backbone system to another point where it is passed to another local system. The most important features of the backbone system are reliability and performance. The technology of the backbone system must be inherently reliable. The backbone technology must also be ubiquitous. It must be possible to connect almost any system to the backbone.

Another important feature of a backbone system is directory synchronization. Directory synchronization means that addresses from one email system appear in others so users in one local system are able to address email to users of another system. Strictly speaking, directory synchronization need not be a part of the email backbone technology itself but some solution must provide directory synchronization.

SMTP and LDAP
One common backbone implementation is SMTP (Simple Mail Transfer Protocol). However, SMTP by itself does not provide any directory technology. In recent years the industry has selected LDAP (Lightweight Directory Access Protocol) to fill that role. LDAP-enabled SMTP implementations such as Netscape Messaging Server and SIMS (Sun Internet Mail Server) are increasingly popular. LDAP technology has been widely adopted and the ability to synchronize directory information from disparate sources and make it available through LDAP has developed into a number of meta-directory products.

Lotus backbone options
For Lotus messaging customers, there are multiple email backbone options including LMS and Domino. It is almost always best to avoid running parallel email routing infrastructures. When migrating from cc:Mail to Domino, you may want to route email across business units exclusively through Domino. Compared with cc:Mail, Domino provides a more reliable email routing system. In a typical Domino system, there can be many more users per server than can be supported using cc:Mail. This means fewer average hops between any two points in the email routing topology and this translates into better performance. Through the cc:Mail MTA, Domino also provides consistency and synchronization of directory information with cc:Mail.