blocking outbound port 25

Forums Network Management Firewall, Traffic Shaping and Net Balancer blocking outbound port 25

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  • #42379

    Hello All:

    I have my default ZS firewall policy on ACCEPT/Forward. It currently does not have any rules on it. Seems that all session initiated from within the lan is open.

    I am wanting to block all traffic from port 25 accept the mail server. What is the best implementation for this.

    I also have VOIP using SIP within the LAN that connects to a external SIP. I have tried to change the policy but it seems to mess this up when I do.

    Thank for any advice.



    If traffic flows inside the LAN there not much you can do to block it. If the mail server is outside the LAN you can easily block access with a firewall rule. Do not change the policy, just create rules that BLOCK the traffic.


    Hello ppalias,

    Thanks for the reply. I wanted to clarify myself on my initial email. I wanted to block all SMTP from within the LAN out to the INTERNET from all devices accept the internal Mail server.

    We caught a machine that was acting like it’s own email server and was possibly sending emails out from within the LAN out to the INTERNET without relaying it to our real mail server.

    is that still not possible? or with that information, there are some hopes.



    It is possible. Create first a rule to allow your mail server only and then a rule to block the whole subnet. Mind the sequence! Otherwise you will block everything!


    Set up a rule on your firewall that blocks outbound TCP port 25, you can then use to test if the rule is effective.

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