Lion, Mail, Rejected Passwords, and the Keychain

Like thousands of others, my Mail installation consistently claims that the server has rejected the password.

Mail has been prone to just this kind of idiocy every since I can remember, but it's 100% hysterical on the subject since upgrading to 10.7.1.

There's nothing wrong with my mail account. (I run the server and besides, mail clients other than Mail have no problems.)

There's nothing wrong with my Mail configuration.

There appears to be nothing wrong at all except, of course, for the fact that it doesn't work.

But then, because I'm running the server, the plot thickens…

• The mailserver's logs not only fail to show that the passwords for my accounts are being rejected, they show no attempts by any client to access those accounts even as Mail continually claims the server is snubbing it. Mail is lying.
  
• The problem occurs only if I attempt to add Mail passwords to the Keychain. If I delete all mail server passwords from my login keychain, and provide the passwords to Mail without adding them to the keychain, Mail connects perfectly and the server logs the connections appropriately. Obviously there's nothing wrong with the password or the server.
  
• Keychain first aid has no effect. Ditto resetting the keychain, deleting my mail configuration and starting fresh, repairing permissions, safe boot, and deleting every cache file that even looks like it might be remotely related.
  
• No matter what, if I try to add a Mail password to the keychain, Mail claims the server has rejected the password without every contact the server.

I've given up for the moment and - absurdly - automated the process of constantly inputting my mail account passwords.

Anyone have any thoughts? I'm really not in the mood to switch to Thunderbird.

Thanks,

Bryan