[Kolab-devel] port 25 blocked by isp, any hints?

[Bo Thorsen]

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On Thursday 31 July 2003 09:32, Maarten Stolte wrote:
> Hello,
>
> I want to run Kolab on my home server, but my ISP, in all its wisdom,
> blocks port 25 to my server.
> Is there any simple program or setup of a mail server so that I can run
> that somewhere else as my main smtp for other servers to connect to,
> and then let it relay or forward or similar the mails to the real Kolab
> smtp server, which I will then run on port 2525 or something like
> that..

A trick that danish ISPs use is to only block port 25 from the outside, 
but not from their mailserver. The reason for this is that you set your 
mailserver as the primary mx, and the ISP mailserver as the secondary. 
When someone tries to send a mail to your machine (because it's primary), 
they will be blocked. The mail will instead go to the ISP mailserver. 
This one sees it's only secondary, and forwards it to your primary one.

This scheme works fine, if the port if rejecting, but gives a ~15 minute 
delay if the port is dropping the packages.

The reason for them doing this is that it's impossible to use any 
mailhosts on the adsl home machines as open relays, since these will be 
caught in the ISP mailserver.

I obviously have no idea if your ISP would support this scheme, but if you 
can get a hold on sufficiently technical support guys, you might pass the 
idea on. Of course, if the reason they're blocking is to force you to buy 
their more expensive professional service, they probably won't see the 
value of this scheme. But usually the reason for blocking port 25 is to 
stop open relays.

Bo.

- -- 

     Bo Thorsen                 |   Praestevejen 4
     Senior Software Engineer   |   5290 Marslev
     Klarälvdalens Datakonsult  |   Denmark
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