I'd briefly gone and done something else while in the middle of typing a message to another member. When I came back and tried to send the message, all I got was a blank page in the body frame of the interface. What's more, there was no obvious way to get my message back - my browser's back button didn't do it in my instance. And of course, it hadn't appeared in my Sent folder either, so presumably it wasn't sent.
 
Because this is a cause of data loss, it ought to be fixed ASAP. Perhaps by showing a proper error page if a message is submitted while the member's account is offline, which remembers the message contents in order that the user may log in again and send what he/she has already typed.

Yes, this was probably due to an inactivity timeout. We'll consider different ways of handling this--either extending the timeout or some type of way to retain the message if you timeout.

No matter how long you make the timeout, somebody's going to be bitten by it. So having a way to retain the message on timeout is the better option.

Why not add a save function which will automatically place the incompleted outgoing message(s) into a 'Pending' folder by default? In this way, the user can save/overwrite the message being composed to that folder prior to leaving for whatever period of time, and protect the loss of any data in the event of a timeout.
 
When the message is complete, then the user can send the message directly from that folder. The completed message can then land in the user's 'Sent' folder.
 
~The Doctor

I've fallen victim to this a few times...and if I didn't print the message prior to the timeout, I could never remember all that I had originally composed.
 
You may already be doing this, but try composing/saving the message in a stand-alone application (i.e.--Microsoft Word, Word Pad, etc.). Then when you are finished, you can do a 'copy/paste' of that message into the message window in Caissa prior to sending.
 
~The Doctor

> Why not add a save function which will automatically place the
> incompleted outgoing message(s) into a 'Pending' folder by default? In
> this way, the user can save/overwrite the message being composed to
> that folder prior to leaving for whatever period of time, and protect
> the loss of any data in the event of a timeout.
 
The problem is that you'd still have to remember to do it before you time out.
 
> You may already be doing this, but try composing/saving the message in
> a stand-alone application (i.e.--Microsoft Word, Word Pad, etc.). Then
> when you are finished, you can do a 'copy/paste' of that message into
> the message window in Caissa prior to sending.
 
Yes, that's another way to do it. A plain text editor would work just as well, considering that word processor formatting isn't going to be preserved when pasted here. But when the message is fairly short, composing in another application can seem to be overkill. Though if you have typed it directly into Caissa, you can still c&p it to another app as a safety net.