Just switched over to Thunderbird instead of using Aspire One Mail (ASM seems to have poor imap capabilities). Here's how to do it.
Download thunderbird using add/remove software
In terminal:
cd .config/xfce4/desktop
nano group-app.xml (You'll have to have nano capabilities installed)
Scroll down to <id>5</id>
Find the line that begins <app sequence="3" acs="email">
Change AME.desktop to mozilla-thunderbird.desktop
Restart computer
To stop Aspire One Mail from checking for new messages:
Open Aspire One Mail
Tools>Options
Uncheck: "Check for Messages Every:" and "Play sound when new messages arrive."
The only thing I haven't figured out is how to get the new message number under the email icon on the desktop to display the new messages in Thunderbird. Also, I'd love to me able to minimize Thunderbird to the tray so it can still check for new messages without being "open." I looked at the minimize-to-tray function, but it doesn't work with my current version of Thunderbird (2.0.0.14). Any suggestions on either item? 