​How you find and copy the extended header information from an email message will depend on what email client or online email access you are using. Most methods can be identified by conducting a search within an online search engine and include the email client/access and “extended headers” or “full headers” as your search terms.
 
There are several recently updated pages online which also include a clear breakdown of the methods used to obtain extended headers for multiple clients.
 
EmailQuestions.com - http://www.emailquestions.com/full-email-headers/  - This site includes a good selection of instructions on how to obtain extended headers for multiple clients and online email services.

SpamCop.net - http://spamcop.net/fom-serve/cache/19.html - SpamCop offers services to help internet users report unsolicited email. Part of their site includes a list on how to obtain extended headers for multiple clients and online email services.
 
Haltabuse.org - http://www.haltabuse.org/help/headers/ - This site is another useful online resource that has information on how to obtain extended header information.