Outlook allows users to remove attachments from e-mail messages. This seems like basic functionality that really needs to be included in Zimbra in order to compete.
The bulk of the storage space is due to attachments, so if users could remove attachments they've downloaded and just save the e-mail message, this could greatly reduce the total file and backup storage requirements.
There's a bug posted, but I was told it needs more votes in order to be addressed. If you think this would help you, please add your two cents.
Bug 233 - Ability to remove attachment from received message
If the system could prompt the user to remove the attachment when they clicked to download it, that would be a great usability feature. There's already the ability to search for messages with large attachments, so users would be able to better manage their space consumption.