What is the relevance of Internet Protocol (IP) addresses in e-mail forensics?

In a general sense, forensics involves the investigation and documentation of a given crime to provide evidence that is typically used for legal action. E-mail is extensively used in personal and business operations to communicate with individuals in a fast and cost-effective manner. However, more frequently than not, e-mail is used as a platform for fraud, deceit, crime, identity theft, or even information phishing.

Tasks:

What are some examples of e-mail scams (for example, Nigerian scams, false credit or account alerts, and dating scams)? What is the purpose of such an e-mail attack?
What is the relevance of Internet Protocol (IP) addresses in e-mail forensics?
What challenges arise when attempting to identify the source of a given e-mail message?