(Please consider this a courtesy more so than a question, but still, please add your advice!)
If you hit "The custom error module does not recognize this error" in Outlook Web Access, and you're in Firefox; then your session probably just timed out! You just wrote a big email!!! If you go BACK, all form fields will be BLANK! AHHHH!!!
Instead of trying to dump postdata, use wireshark on https, or anything else ridiculous: Open a new tab or window in Firefox, and in that new tab/window go and re-login to your OWA site. Go back to the failed to submit tab/window, and right click in the message body area -> This Frame... -> Reload this frame -> Resubmit form! Your email will successfully send.
If this trick doesn't suit you, then I suggest reviewing Lazarus(Firefox addon); remembering to use a text editor and save your email letter; or at the very least skimp by and "Ctrl+C" copy the message body before you submit it. Hopefully this helps anyone looking in Google results wondering how to get their message back.
Now, my 'formal' question to stackoverflow is: what else can you do in this same situation? Precious form data is in memory, the https web application[or at least the session] you were working with is now gone, and plenty of your unsaved work hangs in the balance.
Assuming this is on win32, what can you do to get it back?
e.g. Can you memory dump somehow? Configure a proxy and print the stream? Can Firefox's session data be accessed without closing or restarting the process? Give some ideas please?