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We have had problems before with SMTP port 25 (smtp) - some PC's were sending spam mail throught that port.

So we blocked that and only allowed only outgoing smtp servers we needed.

Howerver, i read there is 587 port (smtp mail submission) which, for example, is used by gmail - i`v unblocked some common ports like http, https, ftp, ssh etc - is it safe to unblock 587 port to any destination ip address? Is/can spam be sent through that port like it was through 25?

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Yes, mail can be sent through port 587, so spam can be sent as well. That being said, most mail server using port 587 will also ask for authentication, so it should be much less of a problem. To be even safer, you could have a local mail forwarding server that require authenticaion and require all mail to go through this server.

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Yes that port is safe as it requireds the user to authenticate before allowing the mail to be relayed.

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That does not mean it is safe. There are lots of spam vectors that use auth. –  benc Oct 21 '10 at 6:53
    
Of course but they would have to obtain the auth details first and if you are running a decent implementation over TLS thats not easy –  topdog Oct 21 '10 at 7:56
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