If all you need is file transfer even the message header is a lot of overhead (how much overhead depends on the client and how many devices handle the message). Mail servers don't always handle large files very well either. Even if they upload correctly downloading can be difficult. It's not uncommon for a single message with a large attachment to clog a mailbox and prevent other messages from being sent/received. That said, I'm not even saying it can't/wont work, just that there's better options for sending files and there are certainly better MUAs than outlook.