In addition to using Proofpoint as a mechanism for an external recipient to initiate a secure message to us: https://support.patriotgis.com/support/solutions/articles/17000149266-how-an-external-user-can-initiate-a-secure-message
You may also use OneDrive or SharePoint to request files. We will use OneDrive for the example below, but SharePoint sites can follow the same process.
1. Open OneDrive online by clicking the OneDrive icon from your system tray, and select View Online
2. Create a folder in your OneDrive. (For this example, we will create a folder named "Upload"):
3. Select the folder, and then click "Request Files" from the toolbar
4. Enter a description and click Next
5. Enter the email address, a message for the recipient, and click Send
6. The external recipient will receive an email where they can upload files to your folder:
7. You will receive an email notification when the file(s) have been uploaded: