How Mac Mail.app sucks, number 317
Mail.app hasn't been able to receive mail since I attempted to send a 200MB video to our neighbors. (Some ne'er-do-wells were messing about while they were at work, right in front of one of our security cameras.) Gmail blocks attachments larger than 25MB, so it bounced and landed in email Limbo, apparently.
The result was the little spinny thing by the INBOX associated with that account, and the activity window showing "copying message" and hanging for about three bazillion years. For future reference, kill Mail.app and remove the .OfflineCache directory in the appropriate ~/Library/Mail/IMAP-this@account.sucks@imap.gmail.com, then restart Mail.app. In Terminal.app, the following works:
cd ~/Library/Mail
cd IMAP-this@account.sucks@imap.gmail.com
rm -rf .OfflineCache
Keywords (for my own searching later): "mac os x" "snow leopard" "10.6.4" "mail.app" "hanging" "gmail" "offlinecache" "imap"