lotus notes 4.6

i have 3 users that can not send and recieve mail. they can log onto the server but when they go to replication tab they are unable to send and recieve mail. They are getting the error message “UNABLE TO save encrypted email the mail file specified in your current location .nsf is not on your workspace or available on the network.” i need to know what this error means, why its called and how to fix it. Please help it would really get me out of a jam!