[Users] Disallow users to authenticate with smtp / hardening Zimbra

[Barry de Graaff]

Hello All, 

I have set-up a hardened Zimbra server, that is, I firewalled pop/imap/http so that is not available. 

Port 443 can only be reached via a VPN. 

So far so good, 

I am still seeing a bot-net trying to authenticate by using username/password combos 
on the smtp port though. So I set up a fail2ban like script to ban ip's that are doing that. 

Please tell me if I am wrong, but if they succeed in getting the smtp credentials for an account, 
they can send out spam and do some spoofing, but they cannot get the users data right? As that 
cannot be fetched over smtp? Even without spamming, one can use the response from Zimbra 
to find out valid username/password combos. Which is bad, but not a big deal, because the VPN. 

Other than using an smtp relay, what can I do to prevent user-accounts being used to auth on 
smtp? I do not really need the feature on this server, but I cannot disable the port, cause then no 
more mail could be delivered right? 

Any suggestions? I still have 465/tcp 587/tcp and 25 opened for smtp. 

Kind regards, 

Barry de Graaff 
Zeta Alliance 
Co-founder & Developer 
zetalliance.org | github.com/Zimbra-Community 

Signal: +31 617 220 227 
Fingerprint: 97f4694a1d9aedad012533db725ddd156d36a2d0 
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