I assume you are referring to the latest version of Mail.app on OS-X?
I have four folks using it without seeing this issue.
It is possible, always, for the Keychain in the user prefs (on OS-X) to be corrupted for the UID/PW in question. I would try another UID/PW combination and see if the issue persists. The e-smith server hasn't given my OS-X users any grief.
regards,
patrick