North American Network Operators Group|
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Re: SMTP authentication for broadband providers
Will Yardley wrote:
If you do that, please put in a corresponding ACL to block port 25 inbound _and_ outbound.My understanding is that in most cases, providers are blocking port 25 outbound to prevent direct to MX spamming from their customers' machines
Otherwise, you just might get bitten by something like spoofed source routing, as several providers have found out in the past.