Very strange networking problem re port 25 traffic…

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    I have a very strange networking problem, its related to exchange communications, but also routing so I thought it would be appropriate for this section.

    We have a client running exchange 2010 and an antispam service (AVG Antispam).

    MX records point to antispam service, who then forward emails on to our server.

    Equally, all outbound emails are routed via avg antispam using a smarthost connector.

    every few days we are facing a problem where inbound and outbound email communications stop working completely.

    We have found that the only thing that will make the comms work again is rebooting the router.

    As soon as that is done, everything works fine.

    Whilst the problem is happening, telneting in to the network from an external computer fails (it just connects with a completely blank screen).

    Also, telneting out to another SMTP server on port 25 also fails – it just times out.

    I originally thought it was the router causing the issue, so that has been replaced with a brand new Vigor router, but still the problem persists.

    In addition, if I open port 26 on the router, and then set up a port redirect for 26->25, it works fine even when the issue arises – this makes me think that internally everything is operating correctly.

    Anyone got any ideas?

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