How can you avoid this message ?

Delivery has failed to these recipients or distribution lists:

The recipient's mailbox is full and can't accept messages now. Microsoft Exchange will not try to redeliver this message for you. Please try resending this message later, or contact the recipient directly.
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Sent by Microsoft Exchange Server 2007

Diagnostic information for administrators:
Generating server:  yourdomain.com

#550 5.2.2 STOREDRV.Deliver: mailbox full. The following information should help identify the cause:

With Box.net you no longer will have a mailbox quota issue becasue you do not need to attach any documents to your files. 

Box.net will integrate with your Microsoft Outlook and replace all your Attachments with an URL address that will empower your collaborators to access the files directly from your Box.

Box acts as an online, virtual file server. You can use it to store all your documents, spreadsheets, presentations, PDFs, Photoshop and Illustrator files, images, audio and video – if you can store it on your computer, you can store it on Box.

Then Box will make it easy to share files both large and small with colleagues and external partners, including all your documents and media content. Box also provides tools to monitor and manage activity across all the users in your Box account. This includes:

• What files are uploaded and downloaded
• Access permissions for individual users and entire departments
• Password protection and time-based restrictions on shared files
• Comments and discussions around files
• The ability to generate and export file and user activity

Say goodbye to guesswork. The Updates tab on Box will keep you informed on all the files you've shared. You’ll always know when a partner, client, or vendor downloads a file, uploads new content, adds comments, or starts a discussion. You can also choose to get activity updates via email. With the Business edition, you can run complete reports on all Box activities over time, increasing oversight and control over vital projects.

Box is completely web-based, so all you need to access your files is a browser and an Internet connection. Using a mobile browser-optimized version of Box on your BlackBerry, you can even view and share files on the go with your Blackberry smartphone.