A few things you can do: 1. Many spammers can switch their IP address but you should blacklist any ip that signs up for an account and spam, it will slow them down at least
2. The 100 cap per day is a good idea but I'd lower it to 5 messages a day, increasing by a couple messages cap per week. They will then likely have to build up time before they will waste the account, but this gives you time to build a log in history of ips. Block anyone who's ip changes often or at least watch those accounts 3. Block certain countries that cause the most spam, china and Russia to start 4. Content filtering helps but keep in mind most spammers are already tricking most common filters since they want to also trick major ISPs like yahoo to accept their mail. Instead, or in addition to a standard filter, start looking for common patterns in their links and images, as well as headers and HTML signature 5. Force a back up email address or phone number as part of sign up. You may want it to be easy to sign up but since you can't really stop a deticated spam team, your goal is to make using your system annoying enough to make them move on to someone else Just some ideas to get you thinking, expect this problem to be ongoing, providers like yahoo spend millions of dollars fighting this problem, there's no quick fix Sent from my iPhone > On Apr 10, 2014, at 6:49 PM, LuKreme <krem...@kreme.com> wrote: > > >> On 10 Apr 2014, at 07:58 , Marcin Szymonik <szymoni...@gmail.com> wrote: >> >> Hello, >> >> We run a free accounts mail server (like gmail) and we struggle with the >> outgoing spam problem. >> Spammers abuse our service by creating accounts and then sending out spam. >> >> It is very easy and free to create an account and we want it to stay that >> way so blocking or removing spammers accounts is not a solition - they can >> easily create many new accounts. >> They use tens of different IP addresses and send from different locations >> and countries so per IP limits really don't work. >> Many of their IPs aren't listed on any RBL at all. >> I feel it would be hard to filter them by message contents - they avoid >> patterns by changing headers (even an encoding), message texts or links if >> they add any. >> >> How can we fight this? > > Require a valid email address to send a confirmation for account sign up. > > Restrict new accounts to sending email only to one destination address. > > Restrict new accounts to a few dozen emails a day and restrict ALL accounts > to something like 100 a day maximum unless they request an increase and seem > legit (this requires human intervention). > > The alternative is to have your system blacklisted as a spam source. Keep in > mind that many mail admins will have their own blacklists, so even if you > don’t get on RBLs or get cleaned up and off RBLs, you may never get off a > particular mail-server’s blacklist. > >> How other free mail service providers block this? > > Some implement Captchas, but as a user I find these horribly annoying and it > often takes me 4 or 5 attempts to ‘solve’ them. I also don’t think they are > at all effective as the botnets and spammers have networks of people solving > them for them. > > > -- > If you could do a sort of relief map of sinfulness, wickedness and > all-round immorality, rather like those representations of the > gravitational field around a Black Hole, then even in Ankh-Morpork the > Shades would be represented by a shaft. In fact the Shades was > remarkably like the aforesaid well-known astrological phenomenon: it had > a certain strong attraction, no light escaped from it, and it could > indeed become a gateway to another world. The next one. >
