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Will blcok port 25 really help to stop spam?

Wondering if block port 25 will really help to stop spam? As far as I know, if i setup the restriction on firewall, inside my network, all clients only able to communicate with my own mail server, and any other on the internet. But wondering if virus not able to send spam using my mail server?

And how usually virus send out spam on client machine? does it use telnet and the client machine act itself as a mail server and send out spam?
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Alan Hardisty
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>If you block outbound port 25 for all client IP's but not your server IP, you are helping yourself because any infected client can use it's own SMTP engine to simply push out spam and get you blacklisted.

I am using windows 7, but I don't see the smtp service ..... ?
and the 2 method you mentioned above are sending through my smtp server, so in this case blocking the port will not help.Does virus also use telnet to send out spam?
Windows 7 doesn't have an SMTP service - your Exchange Server has an SMTP service.

If you get a virus, it may have it's own built-in SMTP service (engine) that you won't know about and the virus will use that to send out emails, hence blocking all internal IP addresses on the router from being able to send out traffic on TCP Port 25 (SMTP), will help stop spam.

A virus won't use Telnet, or at least if it does, it will still be using TCP Port 25 (SMTP) to send out mail.
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> Windows 7 doesn't have an SMTP service - your Exchange Server has an SMTP service.

so how my email client send out email to my mail server?
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> If you install email software such as Outlook and configure that to use SMTP / POP3, then of course it will have an SMTP service installed.

Thank you again. I have outlook installed on both xp and windows 7, but I don't see smtp service in windows services... is it built-in in outlook?
You won't see a specific SMTP service.  It is built-in to Outlook.