> ... The local ISP is intercepting all outbound SMTP traffic and
> routing it to one of his mail servers. ...
In my experience an ISP blocking or rerouting port 25 is very common here in
north america (forging successful return codes is a new twist though:-). In
most cases it's sold (probably accurately) as an "anti-SPAM measure".
> ... I want to rewrite SMTP packets ...
That's the hard way, don't.
The SMTP standard has always set aside separate ports for send and for receive,
and every mail program and server already supports them. Both receiving and
sending on the same port 25 was just a "shortcut" that has been used so widely
folks usually think of it as standard, but it's not.
(Even if you can't use the standard SMTP sending port, IMHO it's much easier to
just change the mail program's configuration than to try to "rewrite" packets.)
> ... to use port 26 instead of 25 ...
Despite widespread documentation, port 26 is often not a good idea. Try the
standard SMTP sending port 587 first. One advantage is every mail exchanger and
mail program already fully supports it and no packet rewriting is necessary.
Another advantage is that most ISPs know what it is and treat it sensibly.
thanks -Chuck Kollars
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