Can't open emails replying to my messages

On many–but not all–emails I receive in reply to a message I sent, I get this error: “Webthority error: session not allowed from this location (8510). Error 403 from server” And then it goes to a blank message.

This only happens on replies and only on iNotes web. Accessing Lotus Notes at my office is the only way I can pull up these emails. Can someone help?