Finally, I got the easiest way to switch between smtp.ku.edu and smtp.sunflower.com: install the extension "Buttons!" https://addons.update.mozilla.org/extensions/moreinfo.php?id=106&vid=122&category=
Cited from: http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Multiple_SMTP_Servers
If you have set up multiple SMTP servers and want an easier way to change the default SMTP server (such as if you have a laptop computer that you use at work and home or on different networks), try the Buttons! extension (http://update.mozilla.org/extensions/moreinfo.php?id=106&vid=122&category=). It will give you a "Select SMTP!" button that you can add to the toolbar to change the default SMTP server at a single click.