There are a number of free plug-ins that you can download for Microsoft Outlook. A software developer sees the need for an additional feature, they develop it and to build up their reputation they offer it free to the small business owner. The developer will make their money through advertising, selling the product to larger users, and potentially selling their business to a larger company.
If you want to send an email attachment typically your internet provider will allow you to send an attachment between 5 and 10 MB maximum; however that can be a slow process, and may fill the recipients’ mailbox.
YouSendIt

YouSendIt is a web-based delivery service that lets users send, receive and track files on-demand. The sender can enter the recipients’ e-mail addresses, attach the file and send it; the free option allows users to send attachments up to 100MB. Using YouSendIt, the recipients receive an e-mail notification with a link to the web that lets them download the file.

You may have seen the following window when you try to access Microsoft Outlook from another program, e.g. when you try and send invoices from your MYOB program.

ClickYes takes less than 5 minutes to download and install and will automatically just clicks the Yes button on your behalf. Once installed you will forget it is there, though you can turn the option off.
from Heather Smith ~ MYOB Certified Consultant