One way to sync mails between different Outlook clients would be to use a free software called Folder Share which helps you synchronize files and folders using a peer to peer network. This essentially means that you can use it to sync any data and not only the data from Outlook.

FolderShare has to be installed on all computers that you use and one account has to be created as well if you do not have one already. The software will ask you during installation if you have an account, if you select No it will automatically ask for registration. You can later on use the data from that account on the other computers as well.

You do need to install FolderShare to at least two computers to be able to sync files which should be pretty obvious. Once it is installed on two computers you click on the ‘Sync My Folders’ button on the master computer and navigate to your Outlook.pst file which can be found in C:\ Documents and Settings\ USERNAME\ Local Settings\ Application Data\ Microsoft\ Outlook if you kept the default path.

Now create a new folder for each other device that you want to sync and name it accordingly (Outlook Sync perhaps). You can select to automatically synchronize the data which is recommended. FolderShare needs to be running on at least two devices to sync the data.

Update: Foldershare has been bought by Microsoft and integrated into Windows Live products. The program is not available anymore as a standalone software to synchronize Microsoft Outlook.