Scam mail beeing sent to my classified posters

I have a classifieds website, and anybody may view the classifieds which people post.

People don't need to be a member or anything to view, or contact a poster.

The problem I have today is that people receive alot of scam-mail from others, who intend to scam them off their money etc...

The process of se开发者_运维技巧nding an email to a poster, is to click "Email Poster" and fill out the form. The form then submits to a php page, which checks against a database that the same message hasn't been sent from the same IP within the last hour, just as a spam measure.

Also I do check so the same ip doesn't send messages on the same classified more than twice, regardless of text, within 10 minutes.

I have some other filters as well, but they don't help 100%.

They still manage to send out one email every hour, my god they are persistent.

Is there any other method to 100% prevent these scam mails? Or do I simply have to live with it?

Thanks

• Add a captcha challenge to your form, such as reCAPTCHA.

• You could ban the IP from sending any emails together if they hit the
  sending limit three times in a row.

• You could require the sender to click a confirmation link that is sent to
  their own email, before the recipient receives their email.

Taking the comments into consideration, I'll add more ways to battle them.

• Require senders to connect with a well know social site (eg. Facebook) to authenticate themselves.

• Block entire countries from sending emails - some countries have a lot more scammers than others.

• Alternatively only allow people to send email to recipients from the same country. (ie. User posts classified from USA, then only IPs from USA can respond)

Are you using CAPTCHAs to prevent bots abusing your system?

Have you considered using something like the Akismet API? Obviously it depends on the kind of spam but that might help.