|I've never used Thunderbird but understand that it works along the same lines as Outlook Express (ie it is an offline email program, rather than webmail).|
I've just wound up my old XP with Outlook Express on it. Here is the step by step on what you ought to do:
First, open OE and go to:
Tools > Options > General tab. Uncheck "Send and Receive messages at startup" if necessary. Apply/OK your way out.
Secondly, from OE go to Tools > Accounts . Single click your account entry and hit Properties button. Now go to Advanced tab and near the bottom, put a check mark in "Leave a copy of messages on server". If you have more than one email address showing, do the same to each one listed. Apply/OK your way out.
Just to be on the safe close OE, restart it and double check that your settings have stuck.
Always pop back and let us know the outcome - thanks