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##

Analysis

Assuming that the deterministic round-robin behavior of the DNS resolution
or layer 4 switching of
mail.bigisp.com gives clients an equal chance of connecting to
each server. If there are *n* mail servers with A records for
mail.bigisp.com the chance of one of the servers receiving a
connection from a foreign host is
.
Assuming that
all users receive an equal number of messages of equal size the
chance of a message from a foreign host being delivered locally
is
.
The number of messages that must be relayed to
another one of the mail.bigisp.com machines is
.
So the probability that each
server must relay is
.
As *n* becomes large this approximates to
,
as
.
Effectively the probability of a message being relayed internally
is 1 and as a result SMTP network traffic is doubled.

This analysis is still valid for a system where users have
different usage patterns, provided that mailboxes are distributed
across the servers in such a way that the servers are equally loaded.
Distribution of mailboxes is discussed in Section 7.

*Horms*

*2000-11-17*