I'm trying to backup (block level) a windows 10 machine with its outlook.
The backup is ok but when I restore the Outlook (I delete its data files before restore), it does not start offline. So in case I want to restore before an unwanted deleting date, I can't count on my backup for outllok.
All docs I find are pst oriented but pst is more for archive and outlook uses a lot .ost files...
Do you have any advice, experience with outlook backup
PST is not only archive, although it can be used for archives. PST files are also used for live Outlook databases when using Outlook with POP3 servers - as opposed to enterprise Exchange or IMAP servers. PST files contain the email as opposed to OST files where they are simply cached from the server. You might also use a PST (as you mentioned) to archive server data to a local file to avoid mailbox size limits or to keep important emails past your company's delete policy.
The best way to back up what's in your OST is to copy to a PST using the Export Wizard or a simple copy operation with an attached PST file.
Since OST files are just cached copies of items that are saved on your mail server, they cannot really be used as backup. Move / Copy what's important to a PST file and back that up instead.