Shortcuts for Switching Mail Accounts Stopped Working

This issue has been raised here (with no responses) and here (solution doesn't work for me) before, but here's the skinny of my problem...

I had three keyboard shortcuts with which to quickly switch the sending account of a newly composed email working successully for a long time. I just switched one work account for another, making the appropriate changes in the System Preferences keyboard shortcuts panel (described here). But now, none of the shortcuts are working.

Any ideas? Is there a preference file I can trash to reset these?

I'm on OS 10.7.5.