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Greylisting [message #116] Mon, 17 April 2006 18:50
Earl
Messages: 4
Registered: March 2006
Junior Member
Are there any plans to implement greylisting?
Re: Greylisting [message #117 is a reply to message #116] Tue, 18 April 2006 16:41 Go to previous message
support
Messages: 919
Registered: April 2004
Senior Member
> Are there any plans to implement greylisting?

We already considered to add a greylisting filter to NoSpamToday! some time ago but
after some research we decided to implement a delay filter instead (see http://www.byteplant.com/support/nospamtoday/howto-use-delay-filter.html for details).

NoSpamToday!'s delay filtering avoids the disadvantages of greylisting and proved to be equally effective in blocking unwanted emails, so it is clearly the better solution.



Customer Support
Byteplant GmbH
Re: Greylisting [message #118 is a reply to message #116] Wed, 19 April 2006 01:01 Go to previous message
Heidner
Messages: 121
Registered: February 2005
Senior Member
I have actually seen several unplanned and undesireable side effects from using the delay filter.

1) Some valid e-mail never makes it through the delay filter. This occurs if the sender is a high volume e-mailer -- such as a newspaper -- I.E. Washington Post. If it takes more than 15 seconds for a response they give up!

2) The delay filter doesn't generate the 550 mail rejected message. I discovered that after implementing the delay filter I saw a SIGNIFICANT increase in mail probes and attempts to send spam. After I removed the delay filter I saw a rather fast decline in probes against may e-mail server and a reduction of inbound traffic. It appears many of the spammers actually pay attention to the 550 message -- whereas if the box just doesn't respond they probe, and probe, and probe again and constantly try sending you spam.
Re: Greylisting [message #119 is a reply to message #116] Thu, 16 November 2006 04:13 Go to previous message
nickvn
Messages: 13
Registered: October 2006
Junior Member
I'm with Heidner - mail server keeps getting probed and they try again, and again and again.

It's now "blocking" 700% more emails - probably because they keep on trying over and over again...


From what I've been told about greylisting, it drops the first connection, then accepts the second connection before processing anything else.
Re: Greylisting [message #120 is a reply to message #116] Thu, 14 December 2006 15:52 Go to previous message
eholton
Messages: 5
Registered: September 2005
Junior Member
If nothing else, it would be good to be able to choose which technique to use.
Re: Greylisting [message #121 is a reply to message #116] Sun, 11 February 2007 14:09 Go to previous message
Roger
Messages: 1
Registered: February 2007
Junior Member
I have used NoSpamToday for over 2 years and have had considerable success.

In the last few weeks I have trialled another SMTP proxy that had greylisting available and frankly the results are spectacular, the actual processed spam is about 15% of what we originally handled, the pests very infrequently return after being told to try later. All this due to greylisiting.

I tried the delay tactic of NoSpamToday and it really never worked for us, once we got to a reasonable delay we started to impact on genuine e-mail, you only need one or two of these valid e-mails lost to cause a loss of confidence in the mail system.

I would suggest that you review your position on greylisting, as an antispam technique it is right up there with the best of techniques.
Re: Greylisting [message #122 is a reply to message #121] Mon, 12 February 2007 14:57 Go to previous message
support
Messages: 919
Registered: April 2004
Senior Member
> I have used NoSpamToday for over 2 years and have had
> considerable success.
>
> In the last few weeks I have trialled another SMTP proxy that
> had greylisting available and frankly the results are
> spectacular, the actual processed spam is about 15% of what we
> originally handled, the pests very infrequently return after
> being told to try later. All this due to greylisiting.
>
> I tried the delay tactic of NoSpamToday and it really never
> worked for us, once we got to a reasonable delay we started to
> impact on genuine e-mail, you only need one or two of these
> valid e-mails lost to cause a loss of confidence in the mail
> system.

Please note that non-RFC conforming MTAs might not be able to deliver
mails regardless if the receiving mail server uses greylisting or is protected by a delay filter.
In addition to that, with delay filtering a legitimate sender gets immediate feedback that an email could not be delivered and can take suitable actions.



Customer Support
Byteplant GmbH
Re: Greylisting [message #123 is a reply to message #121] Tue, 25 September 2007 18:51 Go to previous message
lleachii
Messages: 13
Registered: August 2006
Junior Member
I have also found the need to resort to using an additional SMTP "pre processor" to greylist users prior to being processed by NST.

The "pre processor" software gives the sending server a "451 4.7.0 Greylisted. Delivery delayed." error and then drops the connection. I still use the delay filter after the transmitting server reattempts, this method has reduced the amount of spam that reaches my NST server by over 90% (by review of the statics). I have also noted increased bandwidth, as my server is no longer processing hundreds of spam emails from servers that gave up upon being greylisted.

In regards to Roger, I also experienced a problem where legitimate emails were being disconnected due to the delay filter. I have found that using a 30 second delay with a skip size of 25 kB should be effective for a good amount of spam without hindering legitimate mail. Even with the greylist installed, the delay filter still blocks a daily average of 10% of the spam received (at the NST server).

During my research, the average maximum of spam emails entering my network was about 25 kB, I noticed that a delay longer than 30 seconds caused legitimate connections to drop.

Therefore, I am having extreme success with both the greylist and delay filter; and not to mention, with the DNSBL filter configured.
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