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Domino SMTP error limit before terminating connections

Daniel Nashed  10 February 2020 09:11:15

My friend Harvey and I noticed a lot of brute force delivery attempts on servers to figure out about email addresses.
By default Domino doesn't limit the number of errors until a connection is closed.
But there is a notes.ini entry and setting in config document to define the number of errors.

https://help.hcltechsw.com/domino/11.0.0/conf_definingthemaximumerrorlimitbeforeaconnectionterm_t.html

For larger servers you have to be careful, because that could also prevent newsletters from being delivered, if there are too many users which don't exist.
On my own server I set the value now to 4 ;-)

And if you have more errors -- this includes invalid recipients -- the connection is closed with a temporary error.
But that also means that the mail is not received and the other server would try again.
So you have to be a bit careful with this parameter. I would sent it do a higher value like 20 on larger servers.

421 domino.nashcom.de SMTP service not available, closing transmission channel


Example from log:
10.02.2020 06:21:53   SMTP Server: 185.143.223.xxx connected
10.02.2020 06:21:54   SMTP Server: Mail for morris@csi-domino.com rejected for policy reasons.  Recipient could not be found in the Domino Directory.
10.02.2020 06:21:54   SMTP Server: Mail for mom@csi-domino.com rejected for policy reasons.  Recipient could not be found in the Domino Directory.
10.02.2020 06:21:54   SMTP Server: Mail for buy@csi-domino.com rejected for policy reasons.  Recipient could not be found in the Domino Directory.
10.02.2020 06:21:54   SMTP Server: Mail for abcdefg@csi-domino.com rejected for policy reasons.  Recipient could not be found in the Domino Directory.
10.02.2020 06:21:54   SMTP Server: Mail for az@csi-domino.com rejected for policy reasons.  Recipient could not be found in the Domino Directory.
10.02.2020 06:21:54   SMTP Server: Mail for schmidt@csi-domino.com rejected for policy reasons.  Recipient could not be found in the Domino Directory.
10.02.2020 06:21:54   SMTP Server: Mail for babbar@csi-domino.com rejected for policy reasons.  Recipient could not be found in the Domino Directory.
10.02.2020 06:21:54   SMTP Server: Mail for edith@csi-domino.com rejected for policy reasons.  Recipient could not be found in the Domino Directory.
10.02.2020 06:21:54   SMTP Server: Mail for juliet@csi-domino.com rejected for policy reasons.  Recipient could not be found in the Domino Directory.



Image:Domino SMTP error limit before terminating connections



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