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Concern regarding SMTP security.
I use IP restrictions to prevent relay.
restricting relay to certain IP address's
I was concerned this morning when I noted in the warning log the following entry.

[07/Jan/2009 03:05:59] SMTP: User admin<_a.t_> doesn't exist. Attempt from IP address
[07/Jan/2009 03:06:05] Failed SMTP login from

I considered any login/password based system to have two levels of security.
1. The usernames arent published anywhere two accessible and
2. Robust passwords.

But what I dont get here is that the Ip address involved would normally be restricted from a relay so why are we seeing an apparent login attempt...where the hacker is testing usernames.

I would have thought this would be restricted at the IP level and not wait to reject the login based on USERNAME.


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Until the connecting party actually tries to relay, how will KMS know whether to reject it or not? Relaying is by definition the sending of email to a non-local address through your server, and in this case, no non-local address was given.

If the hacker had succeeded in guessing a username and password, only then would the relay attempt have been stopped. Until that happens, KMS cannot know what the IP is up to. I'd say this is a good enough way to reject relay attempts.

Remember that a legal user will also do SMTP AUTH if the server demands it, even if he/she subsequently sends mail to a local address.

This is the exact reason message submission (RFC2476) was introduced 10 years ago, so that nobody had any reason to do AUTH on port 25, but instead do it on port 587. Kerio doesn't support SMTP AUTH on port 587 though, only SMTP-SSL on the ancient and deprecated port 465.

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I see your point regarding the relay IP sec.

I dont bother with SMTP AUTH (in the relay options) because I limit it to IP's.

Frick, i am going to watch the SMTP debug logs and see whats going on
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