I think that despite the exact nature of the question, all of you have managed to overlook the obvious. Why would e-mail be lost, simply by checking e-mail from another computer. It seems that the likely cause here is that the messages were downloaded using POP and the messages on the server deleted. The most efficient way to restore these e-mail messages would be to import the contents of the .PST file on ‘computer B’, if you will, into the mailbox. Let’s start there. If there are multiple computers involved, it makes the process a more cumbersome but a matter of repetition.
So first, let’s determine if POP is in fact enabled as a service on the Exchange server. If so, then proceed as outlined above. If not, then give the solutions below some consideration. It is important to identify the root cause here because if the user decided to check e-mail again from ‘computer B’ or another computer in the same manner, then guess what: mail is going to disappear again.
For this answer to work properly, note that the server must have already been configured with a deleted items retention policy. If the Exchange administrator had disabled this for some reason, such as very low disk space on the server, then this procedure will not work. But “out of the box,” this solution should not be a problem. So…
If the emails were deleted within the last 7 days you can go into outlook, highlight their “deleted files” folder, then go to TOOLS > RECOVER DELETED ITEMS.
Essentially, by default, to use the Recover Deleted Items command on the Tools menu, you must be viewing the Deleted Items folder. By modifying the registry, this command is available regardless of which Outlook folder that you are viewing. You can view and recover deleted items, including those items that were permanently deleted by using SHIFT+DELETE, for the folder that you are viewing.
You might also be aware that 7 days is the default, but it will depend on your information stores deleted item retention. If you need more time in the future you can move this up, keeping in mind it will increase the size of your store because deleted items are kept around.
Also, you’ll want to pay attention to what the settings for waiting until the information store is completely backed up before deleting items are currently configured.
If the items were deleted beyond recovery, you would need to restore from a backup, and the method depends on what you used for backup tools in the first place. Some software allows for mailbox level restore and some software does not. if it does you would select that user’s mailbox and preform the restore of teh information. If it does not provide for a mailbox level restore (or you don’t own the proper agents) The essentials are that you either restore from a brick-level (individual mailbox) backup using a commercial solution; restore a full backup (with appropriate incrementals/differentials) of the info store to another server and export the mailbox, then merge it back in on the production side; or use a third-party tool to browse the restored info store and extract the data.
The fastest I’ve ever done this was in 2005, and it took 3-4 hours. We restored the info store from backups, then used a third party tool to browse and extract the user’s mailbox.