I sometimes recieve e-mails with empty CONTENT-ID.
Exemple:
In most cases, the empty content-id belongs to an attachement, that is added to the email two times, once as correct inline attachement with a content ID, in the second, the content-disposition is : attachement instead and the content ID is empty.
Is this a mail client bug ?
According to the MIME RFC a Content-ID is optional, but if present should be "world-unique" (which an empty string certainly isn't).
Personally I'd treat empty Content-ID to be identical to a missing Content-ID.