Actually, the arguments being made here are flawed, because they make
erroneous assumptions about high-volume mail architectures.
A large ISP is not just like a small installation times N. Large ISPs
use multi-tiered mail architectures, particularly when anti-spam and
anti-virus filtering is involved.
Typically, you want to take responsibility for the incoming email
immediately; you never want to have to tell the sending server that
it needs to retry later. So you have a bunch of incoming MX servers,
with lots of storage, that just blindly accept email and write it
into a queue. It's possible to build in a bit of intelligence here,
to detect obvious "spam attacks", and just drop those messages on
the floor.
Then, the anti-spam servers process that queue. This is much more
expensive than a blind mail acceptance; therefore, you need a lot
more computing power at this layer. When the anti-spam servers are
done with a message, they hand them off to a mail storage layer,
which is responsible for delivery into a user's mailbox.
Layered approaches allow you to increase the compute capacity on
a task-specific basis.
In general, an ISP deploying a filtering solution clearly either needs
to have a bunch of spare processing cycles already, or they need to
deploy additional hardware along with the filtering solution.
-- Lydia
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