You can configure additional mail stores by entering a partition name and location for each additional store (Server Admin > Mail > Settings > Advanced > Database > Additional Mail Store). The name of the additional mail store partition must be entered in lower case.
If you use any upper case letters in naming the partition, accounts configured to use the alternate partition (Workgroup Manager > Users > Mail > Use an alternate partition) will receive this message when setting up the Mail application:
Mail was unable to open this mailbox on the server. The server error encountered was: Mailbox does not exist.