I have found a fix for this!
First off, what didn't work:
1. Trashing the com.apple.mail.plist in ~/Library/Preferences/ did not work.
2. Trashing the ~/Library/Mail/ folder did not work.
3. Creating a new test user account and setting up the same account in Mail did not solve the problem.
4. Neither repairing disk structure nor repairing permissions in Disk Utility fixed the issue.
5. Deleting the email account in Mail->Preferences->Accounts and re-adding it did not fix the issue.
In fact, no combination of the above steps fixed the "spinning activity wheel next to the Sent mailbox problem".
I confirmed the email server settings both with the ISP, and by successfully setting the account up in Mail on several of the office machines running various iterations of 10.6. I also confirmed that the ISPs servers were viable and even logged into the service's Webmail portal to confirm the account worked properly in all aspects.
Before doing an Archive & Install with the installation disc I called Apple for advice. They confirmed that I had done all the usual troubleshooting steps noted above first and then gave me the following instructions:
1. Delete the application Mail from the Applications folder, drag its icon off the dock, then empty the trash.
2. Insert the OSX installation disc and, in the window that opens in Finder after the disc has mounted, double click the "Optional Installs" folder to open it.
3. Double click on "Optional Installs.mpkg" and after clicking through a bunch of default and informational screens - and entering your admin password once - you'll finally get to a list of the optional installs included with Snow Leopard.
4. Click the little triangle next to "Applications", then tick the box next to "Mail". Click "Continue" and finish the installation.
When done with step four, I attempted to open Mail but was treated to a little dialogue box that told me, in so many words, that this old version of Mail wasn't compatible with the newer version of Snow Leopard that was installed on my machine. I asked the Applecare person about this and she told me she'd seen this problem before. One last step was needed.
5. I went to
http://support.apple.com/downloads/ and downloaded the latest OS combo update from the top of the "Featured Downloads" column on the left side of the page. (If you try to use Software Update instead, be aware that the automatic updates will not check the Mail version number separately from the version of OS X you're running, so manually downloading the combo updater is probably the only option).
Running the combo update on my system (even though I was already at 10.6.6) did the trick. Mail was updated and now opens with no issues. Not only that, but the spinning activity wheel problem is completely gone - I can now send and receive emails without grief.
Good luck!
