This appears to be a relatively recent phenomenon. Some people are seeing it while others are not. Some people have found workarounds, and yet for others those work arounds don't appear to work.
It's quite frustrating for those who are experiencing the problem.
And the problem, it appears, is with Windows Live Hotmail itself.
The symptoms are this: you reply to a message you've received in your Hotmail account, and by that I mean you type in your reply and you hit Send. Later you hear from the recipient that all they got from you was a blank message. When you go to check your Sent Messages, sure enough the message you thought you sent now appears blank.
First, you're not alone.
Second, it's not your fault.
Third, as far as I can tell there's nothing, really, you can do to reliably work around the problem.
And yes, that makes Hotmail pretty useless until the problem is resolved.
Browsing the forums at http://helpcommunity.live.com you'll find several current threads of discussion on the problem.
Some notes from those discussions include:
So the best advice I can offer is to either hang tight, and wait for the Hotmail folks to resolve the issue, or to switch to a different email service.
(*) One sub-question that comes up in these situations is "if it's not my computer, then why does switching computers make a difference?".
Hotmail is not one computer. In fact, I'm certain that it's comprised of hundreds of web servers that all act as the "internet side" of the Hotmail service.
Here's my theory: when you connect to Hotmail, you connect to one of those servers. As more connections from more people are made, the connections are spread out over those servers, but each person connects to one of those hundreds of servers.
Somehow that connection is persistent. That means that if you connect to Hotmail again from the same machine, you'll likely connect to the same specific server that you connected to before. If you connect from a different machine, then you'll likely connect to a different server.
And not all servers are experiencing the problem.
This is only a theory. I'm not saying this is how Hotmail works, but it's a reasonable explanation that I could see being plausible and that might explain why you might see different behavior connecting from different machines.