There is no point in backing it up if you restored it then it would be out of date and soon as you connect a new up-to-date one will be created. A lot of companies don't allow backing it up as its a security risk and breach of data protection in UK the company keeps it secure on the server
Sorry, but did you actually read the OP's post. Your reply indicates you interpreted a few things.
The server isn't the backup, it is primary. If you delete ost file it is rebuilt so no need to backup, with one caveat, you have to assume your mail provider is backing up your mail.
The ost file is just provided so you can work offline.
OK, I'm satisfied. I vote it is solved.
The logic of excluding this file is that this is acompletely unimportant discardable temporary cache file. This file should not be backed up, which is why it is listed in that registry entry.
You can safely remove that registry entry, which will cause that file to get backed up. But why? Where did you get the idea that this file is somehow "important"?
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