MIME setting changing in some apps

I have created HTML email 'signature' files for use in Outlook Express, MS Outlook and Lotus Notes. All work fine, showing pretty text and inline graphic...except on some computers, but not in all instances on those computers. In a couple of instances, when sending from Lotus Notes R5 (using MIME), the email is rec'd as plain text on Outlook Express 6, while the same email is rec'd perfectly by other Outlook Express 6 email clients. In those cases, the multipart/related has been changed to multipart/mixed and the text/html has been changed to text/plain. In one other situation, the inline graphic (which uses a URL address for the graphic) will not show the graphic on the originating computer or the receiver's computer although the text is 'pretty', but using the exact same signature file now loaded on the previously receiving computer, the graphic shows up on both that computer and the previously originating computer. I have searched and searched, but have not been able to determine where the problem is. I assume there is some inconsistent setting somewhere, but I don't know where. I have tried turning off the virus scanner on the receiving computer, but that has made no difference. I need to ensure the signature files are robust - can anybody help me?