Invisible Attachments???

I have been reviewing several hundred emails for a work related project and believed I had all the pertinent information for the project. Then, I forwarded a couple of the emails to another email account so that I could print with a different format, and it turned out that there were approx 3 additional attachments to the emails that, when viewing in Zoho, just showed up as an inline part of the email but the text says "null" so I didn't even think there was an additional attachment. How can I make sure that this does not happen again. It is an extremely serious concern.