bxAutoZip automatically compresses attachments in Outlook and Outlook Express. A great way to save space in your inbox and bandwidth. bxAutoZip is free for personal use:
bxAutoZip is a dedicated add-in utility for Microsoft Outlook and Outlook Express which automatically and independently compresses any files attached to an e-mail message.
Compression dramatically reduces the size of e-mail attachments, allowing much faster transfer times. The result is significant savings in time and money for both the sender and the recipient.