Mime Mail with images larger then 3k

Hello to all, I try to generate greetingcards and sent them as a MIME mail. All works fine if I use Linux. Under windows it works fine as long as the images have less then 3k. But my cards have about 20k and it just doesnt work. Can anybody help?? I never find any information if this is a general problem under windows or if I make a mistake. Thanks Markus I use the mail command with additional headers. On the image I created I use chunk_split(base64_encode("image")) this still works fine, but as soon as I use the mail( ) function it doesn't work.