On 9/10/23 08:22, Bill Cole via Postfix-users wrote:
On 2023-09-10 at 04:28:38 UTC-0400 (Sun, 10 Sep 2023 15:28:38 +0700)
Jesper Hansen via Postfix-users <seabreeze...@gmail.com>
is rumored to have said:
I simply sit on their fiber and does not relaying anything through them.
Yes, but your packets traverse their routers and can be hijacked.
To be a bit less obscure, the ISP is applying Destination NAT -- also
called DNAT or redirection -- to your outbound SMTP traffic, rewriting
the destination IP in your outbound port 25 traffic to point to its
outbound mail relay.
Your only fix for this is to get your ISP to stop
hijacking your port 25 traffic. They are doing that to prevent spam
coming from their network, which is good. They may also be doing that to
provide incentive to individuals like yourself to buy a more expensive
grade of service that allows mail to flow unmolested.
There's another possible workaround, if the ISP won't budge (or even if
it does and you still find yourself blocked based on your providers IP
reputation): some service providers may let you purchase outbound mail
relay service; you'd send outbound to them via an unblocked port like
587 and they'd relay via port 25. Shop carefully as many of these
services also have reputation issues.
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