This is incredibly frustrating. At my work, I have two Exchange accounts for my job, one personal, one for receiving support requests. I love the Mail.app for Lion, because it has a lovely Archive feature that I use extensively to keep an archive of information on the various errors and issues that I encounter and support. It's my own little hivemind of information. The problem is that at random points, the Archive and folder system goes haywire. I know it's happened because if I look at the list of boxes and folders, I'll see there is no longer an accessible Archive folder for the affected account. If I'm not paying attention and try to archive an email anyways, I receive the following message: If I go to Mac HD > Users > username > Library > Mail > V2 > affected mailbox folder, I can see all the other .mbox files, but not one named Archive. Alternatively, Archive.mbox may appear, but it will only contain Info.plist. Here are the steps I've taken to try and fix this, with their results. Delete and readd the account. RESULT: Temporary fix. Rebuild mailbox. RESULT: No change. Restart computer. RESULT: No change. Close and reopen Mail. RESULT: No change. Backup 'working' Archive.mbox, and replace when it vanishes or is corrupted. RESULT: Sometimes works, but not permanent or reliable. Disable storing of Trash, Junk, and/or Drafts on server. RESULT: No change. Does anyone know what's going on?