Outlook PST incremental backups ( Block level ) is possible in Windows Agent based backups.

If you install the CBT driver it should do an incremental of the entire drive or whichever backup method, you choose - Veeam Changed Block Tracking Driver - Veeam Agent for Microsoft Windows Guide

You can backup PST files, but you'll have to do volume level backups if you want to backup at the block level.

Please keep in mind that Outlook OST files on the other hand are excluded from backup and can't be restored.

• The Veeam CBT driver is designed to track data blocks that changed since the last run of the backup job. If you work with a file in an application that downloads the entire file into RAM, after you save the changes, all file blocks will be overwritten. Veeam Agent will backup such a file as a whole upon the next run of the backup job.
• https://helpcenter.veeam.com/docs/agentforwindows/userguide/backup_cbt_driver.html?ver=60&_gl=1*1rgf6ah*_ga*MTkzMzM5MTkzOS4xNjgyNzU3MjY4*_ga_PMJS81E58L*MTY4MzU3MDg3MS4xNS4xLjE2ODM1NzI3MzMuNjAuMC4w

Does the highlighted sentence means: If suppose Microsoft Outlook (PST)  is open , and backup job starts  then it will take incremental backup of pst outlook file and suppose user close the outlook application and  next backup schedule starts  it will be a Full backup and not the  Incremental backup.