I created an company-wide email signature template on my MAC starting with an RGB vector logo from Illustrator--converted to a PNG. When some users send an email with the signature template, the logo image looks faded (as if its a watermark) and blurry. This is not consistent with all users, since some signatures appear to be ok. Now, some of the logos placed into a Word document on the PC are faded. When the document is opened on a Mac, the logos usually look fine. --So far the Word documents prints ok, printing isn't an issue with the emails.
If the email footer is embedded in through individual user Microsoft Outlook preference panel the user has a setting that can render the quality of the image you are placing. The following link describes the problem and the solution to fix this issue by using an image editor - Outlook blurry or fuzzy images Hope this helps
Another way around is to get your IT department to invest in a bit of software (cant remember what its called) that they can add the footer (across the server end) - our company looked into it but might be a little pricey. It can add user group rules for different departments if you want them to have a footer with targeted messaging.