Exchange Won't Mount or PST Corrupted? First Steps

Exchange databases and Outlook PST/OST files are dense, structured containers, and both corrupt in fairly predictable ways. A database that won't mount or a PST that Outlook refuses to open usually means part of the structure is damaged, not that the mail inside is gone. What you do in the first few minutes affects how much of it is still recoverable.

Check the exact error Exchange gives when it fails to mount the database, since it usually names the problem: a dirty shutdown state, a missing or mismatched log file, or a specific consistency check failure. A dirty shutdown after an unclean stop often just needs the outstanding logs replayed, which is a very different situation from a database eseutil reports as genuinely corrupted.

Exchange's own repair tool, eseutil in hard-repair mode, can bring a database back to a mountable state, but it does so by discarding whatever pages it can't repair, which can mean losing data permanently. Make a copy of the database and log files before running any repair tool, so the original is preserved if the repair doesn't go as hoped.

A PST that Outlook reports as an invalid or corrupted file, or that is stuck at Outlook's maximum size, usually still has most of its content intact in a structure that can be read directly rather than through Outlook. Scanpst.exe, Outlook's built-in repair tool, can fix minor issues, but on a badly damaged or oversized PST it can also fail partway through or discard folders it can't parse. Work from a copy, not the original file.

An OST file left behind after an account was removed, a profile was deleted, or a mailbox migration went wrong often still holds a locally cached copy of the mail. Even though an OST isn't meant to be opened on its own, its cached content can usually be extracted and converted into a usable PST, without needing the original account to still exist.

Do not delete and recreate a mailbox or database to "start clean" before recovering what you can from the damaged one. Do not run repair tools directly on the only copy of the file. Do not assume a purged or deleted mailbox is unrecoverable without checking retention and deleted-item settings first, since recovery windows are often longer than people expect.

Ask a specialist when eseutil reports the database as genuinely corrupted rather than dirty-shutdown, when a PST or OST is too damaged for Outlook's own tools, when the mailbox data is business-critical and you want to avoid the data loss that hard repair can cause, or when a repair attempt has already been tried and made things worse. Recovery works directly from the database or file structure, without needing Exchange or Outlook running, and we say what looks realistic after an assessment.