CommuniGate Pro allows account users to access Mailboxes in other Accounts. See the Mailboxes section for the details.
Many popular IMAP clients do not support foreign Mailboxes. There is a workaround for IMAP mailers that use the "subscription" scheme. Subscription is a list of Mailbox names that the mailer keeps on the server. Usually, mailers build the subscription list when you configure them for the first time. Later, they show only the Mailboxes included into the subscription list.
By using a different IMAP client or the WebUser Interface, a user can add a foreign Mailbox name (such as ~sales/processed or ~public/news/company) to the subscription list. This will make the old IMAP client show the foreign Mailbox along with the regular account Mailboxes, and the user will be able to work with that foreign Mailbox.
Some IMAP clients do not support foreign Mailboxes at all. To let those clients access shared Mailboxes in other Accounts, Mailbox Aliases can be used.