Imposing email restrictions on a web page

To avoid spammers, what's a good strategy for imposing restrictions on users when sending emails from our site? Account limit per day on individual IP addresses? Sending emails? Domains? In general terms, the recommended numbers will also be helpful.

Our users can send emails through our web page. They can register and log in, but they are also allowed to do so without logging in, but with captcha and with a field for senders' email. Of course, there is a headline: "The user sent you the following message". Limiting the use of spammers is perhaps not a big problem.

Any comments on what I am doing would be greatly appreciated.