Can you suggest a DNSBL that does this?
No, I can't. All I know is that James' (http://james.apache.org) users
periodically complain that James is broken when in fact their dynamic IP is
being blocked, so I figure _someone_ has a list.
For example, we get several hundred messages
from east.verizon.net every day.
Could there actually be legitimate MTAs in that block?
Well yes, if you were a small business you might reasonably be running your
own MTA
to handle mail efficiently for the half dozen people who use the 2 or 3
PC's in your home office.
The workaround would be as simple as having them route through their ISP's
SMTP gateway.
If we are careful to distinguish between blocking outgoing connections to
port 25 and listening on port 25 just anyone with an IP in an ISP's block
will, by definition, have an ISP who can provide an outgoing SMTP relay.
Letting people also directly listen on port 25 lets them host their own
SMTP mail domain and implement all the MTA functionality they could wish
for, without creating an empty playground for spam-bots.
d.
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