Forum/Blog/E-mail Spam

Simply put - spam is spam.
The first rule of the ethical SEO is: do not use spam methods.

The second rule of the ethical SEO is: do not use spam methods!

Here is a more technical explanation of why the spam is bad.

* Spam pisses off forum readers, blog readers and owner, e-mail addressee.

* Spam links on forums or blogs are useless in the terms of SEO, because forums where you can freely post spam comments are usually of very low quality and thus such links won't give you any link juice. On the other hand, quality resources are usually human moderated and your spam comments won't pass through anyway.

* Someone can abuse your spam activity to SpamCop or other anti-spam freaks.

* Spamming requires abuse-proof hosting, abuse-proof domain registrar, abuse-proof payment processor and abuse-proof conscience.