Message limits and slow links
Exchange Server 2003 in conjunction with Outlook 2003 are particularly adequate for branch office scenarios, because they provide nice features, such as offline mode and traffic compression.
Anyone who is responsible for managing a link with reduced bandwidth is probably concerned in keeping large traffic away from the link. That's why setting message limits is considered a good practice in your messaging infra-structure. The main problem, till now, was that those limits were only enforced at the server side, meaning that if a user that is only allowed to send 2MBytes sends a message larger than that, the message will travel all the way till the server side, before being rejected.
The good news is that Microsoft has recently released 2 hotfixes, one for Exchange and the other for Outlook 2003 which allow overcoming this limitation.
Here are the KB articles: