Final answer:
The most reliable method to trace the origin of an email is through Email header analysis. While IP address check, Domain tracing, and Packet inspection are also useful, the email headers provide the most direct evidence of the email's path and origin.
Step-by-step explanation:
A valid method to trace an email back to its originating server and email account is a) Email header analysis. This technique involves examining the email headers, which contain detailed information about the journey of an email from its origin to the recipient. Email headers can reveal the IP addresses of the servers through which the email has passed and also give clues about the email account that sent the message.
Other methods mentioned like b) IP address check, c) Domain tracing, and d) Packet inspection are also useful under different circumstances. An IP address check might give you the location of the email server, domain tracing can help identify the owner of the domain, and packet inspection could be used in real-time to analyze the contents of network traffic associated with email transmission.
However, it's important to note that these methods might not always yield definitive or accurate results due to various factors such as the use of VPNs, proxy servers, or email services that protect user privacy.