If you are using an email program like Outlook, Windows Mail, Windows Live Mail, Thunderbird, et al., you can open up the email, hit File>Save (or Save as File) and the end result will be a file type that ends in .eml.
This is the only way to save the whole email including the hidden metadata (source). Saving as the file extension .eml, also preserves any attachments.
However, if you are using a web browser to access your email online (Hotmail, Yahoo, AOL, TWC, Insightbb, etc) your browser cannot create a .eml file but you can save the email as a MHT or PDF which will be minus any attachments.
Internet Explorer has "Save as" and then you can pick MHT as the File save type. The MHT capability is built into Internet Explorer. Firefox doesn't not have the "Save as MHT" option unless you install an addon called unMHT.
The last option is saving the email as a PDF. To save anything that is printable (web pages, emails) as a PDF file, you need PDF creating software. The one I use is PDF Creator (
pdfforge Download).
It's free but you need to opt out of the extras you don't need or want. To opt out of these unwanted extras, choose "
Expert Installation" when you see it and when you get to the screen where you select components, click the dropdown and select "
Compact Installation". This tells the installer to install PDF Creator and not the other trial program that comes along with it. Proceed with the install until you come to the screen where two boxes are already checked that will install two more unwanted things (AdaWare?).
UNCHECK those two boxes and finish the install.
What PDF Creator does is installs a virtual printer in your printers list called PDF Creator. Then to save whatever you are viewing as a PDF, you print as normal but on the screen where your default inkjet or laser printer is already selected, click the dropdown and choose the new additional printer called PDF Creator. Go through the save screens and viola - you have created a PDF. I use PDF Creator almost daily.