I've experienced described above problem and with some help solution was found.
Problem description: When a 'specific' mail is forwarded it appears "cut" and with no attachments. It happens with Outlook Express and Thunderbird - POP3 accounts configured. However when you open the mail in WebUI everything is just fine - message is whole and attachments are there. When LDAP account is configured, again everything is fine.
'Specific' mail description: The problem mail is with "Content-Transfer-Encoding: 8bit" and contains 2 photos.
Flow description: The 'specific' mail comes to mail server in 8bit, then user Ani downloads the message via POP3 in her Outlook Express. She decides to share this photos and forwards the mail with some text in the body.
a)But her Outlook Express uses 7bit "Content-Transfer-Encoding" and the fun begins.
b)Zimbra mail server does not support 8bit "Content-Transfer-Encoding", so it sends message in 7bit.
People(again using POP3 accounts) receive message cut in the middle of the text Ani had added and with no attachment. They are trying to read message which claims to be 7bit encoded, but actually is 8bit encoded and some certain bits order, create a STOP signal and message ends in the middle.
Solution description: Teach your mail client how to send messages.
Outlook Express Tools→Options→Send→Message Format
enable Thunderbird Tools→Options→Composition→General→For messages that contain 8bit char ...
I know Flow description maybe is not correct, but Solution description works

. I'm not a SMTP guru so if someone can explain this behavior/problem/solution it would be nice.