[polshaw]

I think you could make it not worth it for spammers fairly easily-- only allow sending to one recipient, captchas, rate limiting by IP etc.

[reacweb]

These 3 examples are easy to work around using zombies. Even whitelisting only people who have already written to you can be attacked.

[ruport]

so, from your point of view, are there any better ways to prevent automated spamming ?

[peterwwillis]

Look into how Cloudflare handles detecting abuse of bulk automated http requests (hint: browser trickery). RBLs will help, as well as basic rate limiting. You can also do heuristics on the rate of mails going out from multiple users in similar ways and impose a timeout on users when you detect something 'feels spammy'. Basically you just want to make it so annoying to send spam that they'll use someone else's service. You can also use obfuscation techniques to make it difficult for spammers to determine the method to use to automate requests.

[ruport]

hm yea, nice idea.. i already thought of such solutions. maybe i gonna realize it.