I think the real solution here is to get the live mailbox off a PST file. Only POP3 needs the mail to go to a PST file, both IMAP and Exchange (MAPI) use an OST cache, which are automatically excluded at the shadow copy level because everything in it is also saved on the mail server. Once you switch to IMAP or MAPI, you can set up an auto-archive rule in Outlook to regularly move old emails to a PST archive which will then only change and get backed up at the set archive interval.
Without this or BTRFS/ZFS as options, the next best thing I can think of would be to exclude the PST from urBackup, then use another local solution such as Backup and Restore (Windows 7) to backup the PST exclusively to a cheap USB HDD. That will get this redundancy off the network. You would still probably want to set up archiving though.
@jcmlny that guide link works fine for me.