Because it raises some very "interesting" issues about just
which server a particular client should be bound to. The
network-nearest available one or the one associated with the
same organization as the client are typical possibilities
(along with the somewhat vague "email address") and, if the
wrong one is chosen, it may be inaccessible.
The user still needs to specify their email address. I get my IP service
from comcast but I do not use their mail service. This is why I do not
see DHCP as a mechanism.
If I have pbaker(_at_)verisign(_dot_)com there is only going to be one set of
servers for incomming mail for that address, the place for POP3 and
IMAP4 services is obvious.
Traditionally it was possible to choose the outgoing mail service. That
option is effectively closing due to spam control measures. But even if
the email service has a choice of outboud email relay it is a limited
choice and this does not affect the means used to advertise each relay.
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