Users from one forest can share PIM data with users from another forest by replicating user account information from one forest into the other forest, including SIDs for each user account (referred to herein as a synchronization process). Using this replicated information, the sharing user wishing to share PIM data across a forest boundary can set a permission to allow the cross-forest user to access the PIM data. The sharing user adds the SID of the cross-forest user to the ACL of the sharing user's mailbox. When the cross-forest user wishes to access the PIM data from the sharing user, the cross-forest user's PIM application obtains the mailbox identifier information of the sharing user from the user's forest. With this mailbox information, the cross-forest user's PIM application can then send a request to the sharing user's mailbox server to access the PIM data.