I've always found that Exchange's webmail system, aka "Outlook Web Access", to be extremely buggy regarding remembering who you are. It sets sessions cookies with a pretty far-away timeout but regardless of whether I'm connected via a VPN or from the public internet it always delights in logging me out as quickly as it can. Well, I've worked out a work-around for these all too frequent occurrences. Here's what you do:

• When you are provided with the login prompt, login.
• Click the Back arrow in your browser until it asks if you would like to resubmit the form again.
• Click Cancel.
• Click Back again. This will take you back to the original edit window with the fresh empty (or if it was a reply, the original quote) edit page ready to go.
• Click Forwards. It will say that it needs to resubmit a form in order to proceed.
• Click OK / Accept / Aye Cap'n, whatever is the appropriate response to the affirmative.
• Your message has now been sent, just like when you originally clicked Send.

Hopefully this will help some others who are also loosing their hair over this stupidity.