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 to request files
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: