I am developing an email client in Python.
Is it possible to check if an email contains an attachement just from the e-mail header without downloading the whole E-Mail?
"attachment" is quite a broad term. Is an image for HTML message an attachment? 
In general, you can try analyzing content-type header. If it's multipart/mixed, most likely the message contains an attachment.
Is it possible to check if an email contains an attachement just from the e-mail header?