I have seen this in an exchange environment with errors in configuration. You do not say if you are using Pop3, IMAP or Exchange as your server. What you can do is run standard mail tests and see if you can duplicate the issue on another machine – this will narrow it down to either the account, the machine or the server.
Rule each of them out one at a time and try and pinpoint your issue. Please reply back with your system information like Email Server type, OS and SP.