I'm a newcomer to Thunderbird, using v. 3.1.7. I have it set up to check four separate accounts. When one of them, MobileMe, gets a new message, the "MobileMe" subfolder of the unified Inbox indicates that it's there (it turns boldface blue and sports a "1"). But the message itself does not exist in the unified Inbox. It only exists in the MobileMe inbox.
Why would this be?
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Replying to my own question: I found this answer elsewhere on Mozilla Messaging. I followed David Bienvenu's suggestion for a fix and it worked. (Note Gus Michel's clarification at the bottom.)
I hope it doesn't happen again, but it strikes me that it might--there's a bug in the program somewhere. This glitch has affected all three of the Macs I just installed Thunderbird on.