Tar Pit - outgoing emails

[#1]

I thought Plusnet had a tar pit policy which drastically slowed outgoing emails if you sent them to a big mailing list.
Having read this by Bob over at TBB

Quote

this particular issue <<hot mail blocking emails>> arose after one of our accounts got compromised by a spammer. They used the SMTP auth feature of the account to send in excess of 3 million spam emails over a single weekend before we realised and were able to stop it

I have a question. This is equivalent to about 20 emails per second - so essentially the line must either have been saturated all weekend by sending individual emails or they got round the tar pit somehow.
Perhaps someone more knowledgeable than me could explain.

[#2]

If a spammer using a botnet sent the emails then it would take a lot more than 20 messages per second to saturate the bandwidth they have at their disposal. We do have tarpitting measures in place, however they're only effective to a certain degree and don't failsafe us against every eventuality.

[#3]

Doesn't the tar-pitting only apply to multiple recipients of a single email?
Will iron-port improve things when it takes over from relay.plus.net?

[#4]

Quote from: jelv

Doesn't the tar-pitting only apply to multiple recipients of a single email?

There's a very top-level explanation on our website here. The following is from a *very* old discussion that I had internally when we altered the way that tarpitting worked. It's probably changed now and will be irrelevant anyway once we've moved to IronPort for outbound mail.

Quote from: Somebody

assume 10 emails each to 40 recipients Each email would have a 0.25 second delay increasing by a factor of 1.015 each time for 36 recipients. Each email is therefore delayed by 11.82 seconds After email 2 there would be a 0.5 second delay between each response increasing by 1.05 each time for 8 mails. This adds in an additional delay of 4.77 seconds This adds up to a total delay of 8 * 11.82 + 4.77 = 99.33 seconds. Going on the second example and sending 20 emails to 20 recipients with exim would take: 18 * 4.48 + 14.07 = 94.76 seconds If someone tried to send a single email to 400 recipients this would take 1hr 40 mins Or if someone tried to send 400 emails to 1 recipient then this would take 19 hrs 25 mins (assuming that the SMTP connection was not closed between each mail)

I kind of lost interest with tarpitting after trying to get my head round that!

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Will iron-port improve things when it takes over from relay.plus.net

Yes, definitely.