How to detect forged From and Reply-to addresses from your own domain

Glenn Steen glenn.steen at
Tue Mar 9 08:20:43 GMT 2010

On 8 March 2010 18:17, Michael Masse <mrm at> wrote:
>>>> Mark Sapiro mark at> 3/6/2010 11:19 AM >>
> My ISP redirects all port 25 connects to its own servers so even if I
> know what I'm doing, I can't use your MTA for my outgoing mail for this
> account.
> I've heard of ISP's doing this with port 25, but not 587 which is a standard
> alternative port for smtp submittal.  Are there ISP's redirecting 587 as
> well?

None that I know of... And doing so would entirely defeat the prupose
of "blocking" port 25... Doing that (as an ISP) is a service to us
all, so that mass-mailer crap just can't function. IIRC the swedish
ISPs were among the first to do this on a large scale... Made
move from one of the worst "spam sources" to one of the least
Since the alternative port come with an authentication setup, it fits
the purpose perfectly... And hence the ISPs just plain don't have any
valid reason to block it.

> -Mike

-- Glenn
email: glenn < dot > steen < at > gmail < dot > com
work: glenn < dot > steen < at > ap1 < dot > se

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