The URL at
http://spameatingmonkey.com/lookup/212.23.3.141 explains what the problem is. Your IP address has been blacklisted by an anti-spam blacklist called spameatingmonkey.
In your particular case, your IP address has been added to a backscatter spam blacklist. Backscatter means that oyu are not deliberately spamming; instead, either you or your ISP is indirectly spamming, by sending "bounce" messages to spam email. The buil-in Mac email program has a "bounce" command, but you should never, ever use it; spammers always forge the From: addrss, so if you bounce a spam message, you will end up sending it to the poor schlub whose email was forged.
Similarly, ISPs must never bounce spam, because that just sends the spam to the person whose email address was forged. If you are not bouncing the spam, call your ISP and see if they are.
If you click on the link in the message you posted, you will see a place you can click on to ask that your IP address be removed from the blacklist.
