Getting back to J.Cook's original requirements it would seem the problem is the reception of large files via email from third-parties, with whom he has little control of.
Now in the past I've had problems with the sending of large files and jumped on Ipswitch's MOVEit Ad-hoc Transfer, because this has plug-in's that integrate with MS Outlook and web browser. Once configured the user see's little change and still goes about using these tools in the normal way. However, under the hood the MFT is working, so that when files over a specified size are attached the MFT moves the file to a server, and attaches a URL to the email instead. The recipient merely needs to click on the attachment to initiate download of the attachment.
This is great for sending files out, but not for reception as you have no control over third-parties and what they are running on their systems. Short of demanding that they subscribe to a third-party exchange service there is little that can be done to prevent them using email.
Therefore I suggest that you create a dedicated email server/account for this traffic, potentially on a different domain to your normal end user email accounts. This server can be given different parameters concerning size limits, enabling items received to be formally checked in and distributed within your organisation in a controlled way. You may also wish to set up an email pre-processor, on your normal email service, that captures emails with particularly types of attachments and redirects them to your specialist email server, users needing access to such captured emails can either use a web-service or a hotline to request delivery. The reason for suggesting a dedicated server etc. is for simplicity (it only needs to support SMTP) and to protect end user email services from the side effects of receiving complete VM images as a series of attachments...
Obviously as some have pointed out, you could still cater for those suppliers who are able to use a website etc. through locally hosted solutions such as that provided by Sharepoint etc.
Biting the hand that feeds IT
