I regularly post in the Joel on Software forums. The JoS forums have a very simplistic interface, but they do have allow you to send an email to a poster who has provided their email address.
The thing is, there’s nothing stopping spammers from making use of that. The forum’s presumably-quite-effective Bayesian filters (there is usually very little spam) don’t seem to check emails out, and there’s not even a CAPTCHA.
I received my first spam email through JoS about 2 months ago, but none since. Until this week.
I use Gmail for my email, which is usually quite good at picking out spam, but, presumably thanks to the JoS connection, these spam messages are getting through just fine.
If you are offering users an option to send emails to third parties, you should take steps to try and prevent spam. Otherwise, spammers will take advantage of your name, your service, and your bandwidth to send emails – potentially even result in you ending up being blacklisted. And not entirely unfairly.