For some time now I've been having a lot of problems with Mail.app, including frequent crashes. I've looked at the stack traces and the list of open files (courtesy lsof) and can't believe how complicated this program has become. It has 20+ threads running typically and is touching files all over the file system. I have two external mail servers. One is GMAIL, the other is an Exchange server. If I disable both of them the problem goes away. But it appears if either is enabled. I have thousands of messages in each, but they are both relatively static. I've scoured the forums and have seen a lot of people complaining about Mail causing battery life problems, especially with gmail, but that's all limited to initial sync, not to continued use. I do not see these problems with Thunderbird or even Outlook. It seems to be specific to Mail.