How do I rename the Outlook OST-file? And is something missing?

Question

After a major breakdown in my Outlook (lost connection to my Gmail) it is now up and running again.

However, two things remain:

1.

The new OST-file is for some reason named "@gmail.com(2).ost" Where does the "2" come from? I can´t rename it. If I rename it to "@gmail.com.ost" in the file explorer, and reopens Outlook, Outlook is totally empty and it starts again to download my emails from the mailserver, and again naming the OST-file "******@gmail.com(2).pst". If I go to the account settings in Outlook, in the Data tab, I can´t rename it - name is "locked". How do I rename it? There's no reason that it has the unnecessary "(2)" in the title.

2.

After reinstalling my email account the OST-file has shrinked from 2.5 GB to 1.8 GB. I can´t tell if something is missing, it doesn't seem like it. Is it because Outlook has deleted empty space/compressed it?

Answer 1

The “(2)” in the OST file name is created automatically by Outlook when a file with the original name already exists or previously existed. Outlook will not let the active OST be renamed from inside Outlook, and if it finds a conflict or a problem with the original file, it generates a new cache file and appends a number, such as (2), to keep the name unique.

Renaming the OST manually in File Explorer while Outlook is using it causes Outlook to treat it as missing or invalid. Outlook then creates a fresh OST again, which is why it re-downloads all mail and recreates ******@gmail.com(2).ost. This behavior is by design.

If the goal is simply a clean OST name, the supported approach is:

• Close Outlook.
• Go to the OST folder (for example C:\Users\<username>\AppData\Local\Microsoft\Outlook).
• Rename the existing OST to .old (for example, ******@gmail.com(2).ost → ******@gmail.com(2).old).
• Start Outlook and let it recreate a new OST and fully synchronize.

This is the same pattern used when rebuilding an OST: Outlook creates a new file and downloads data again from the server, and the name is controlled by Outlook, not by manual renaming.

2. A smaller OST size after recreating the profile or OST does not necessarily mean data is missing. When an OST is rebuilt, Outlook downloads the mailbox content from the server into a new, clean file. Old OST files can contain:

• Corrupted or orphaned data structures.
• White space and internal fragmentation from years of changes.

Rebuilding removes that overhead, so the new OST is often smaller even though it contains the same mailbox content. The documentation explicitly recommends rebuilding the OST when there are synchronization issues, which results in Outlook downloading information again from Exchange (or, in this case, the server) into a fresh file.

To verify nothing is missing, compare folder contents (especially older mail, calendar, contacts) against the server view (for example, webmail). If counts and items match, the reduced size is due to cleanup/compaction, not data loss.

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References:

• Synchronization problems occur in Outlook and in Outlook Web App
• Synchronization issues between Outlook and Outlook Web App