Hello, felix@xxxxxxxxxxx a écrit : [...]
For instance, I know that when a client connects to a remote server's port 25, when the server accept()s the connection, it assigns a new temporary port to that connection.
Huh ? What are you talking about ? Aren't you confusing with FTP ?
Suppose I have an iptables rule which sends all outgoing traffic with a destination port of 25 down the static pipe.
You need more thant a simple iptables rule. Iptables will just mark the packets and do some NAT if required, you need also advanced routing to route marked packets through the proper interface. See the LARTC howto. Do not forget to disable source address validation (rp_filter).
[cut irrelevant stuff]
Incoming to the local SMTP server doesn't need any attention, right?
Reply traffic of incoming connections must be routed through the same ISP, because the other ISP may consider it as IP spoofing and drop it. This also requires advanced routing.
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