Mail cannot access your mailboxes because they are locked

I have a user who is NetBooting, his home directory is on an SMB server. When he opens mail (POP account) and authenticates with password, the gear spins and he may be able to read one or two messages then he get the "Mail cannot access your mailboxes because they are locked" error. I can go into ~/Library/Mail and delete the Envelope Index file and then restart Mail and reimport his mail boxes but, then the problem start again. I've even moved the whole ~/Library/Mail folder out, delete his com.apple.mail.plist in ~/Library/Preferenes and started from scratch (not reimporting mail) but, the locked mailbox problem keeps cropping up. Any suggestions?