Can I delete an OST file without losing email messages?

My OST file has grown to 38GB and Outlook 2016 takes forever to open. I read that deleting the OST forces Outlook to rebuild it smaller.

Before I do anything, will deleting the OST wipe my emails? The account is an Exchange mailbox at work. I cannot afford to lose anything.

Short answer for your case: yes, it is safe. An OST is only a cached copy of a live Exchange or IMAP mailbox. The real data sits on the server. Delete the OST and Outlook downloads a fresh copy on the next start.

The one situation where deleting an OST destroys data is an orphaned OST. That happens when the mailbox or account it belonged to no longer exists on the server. In that case the OST is the only copy and you must convert it to PST with a tool like SysTools OST Converter before touching it.

Since your account is live, do this:

1

Confirm everything has synced. Check the status bar says All folders are up to date and the Sync Issues folder is empty.

2

Close Outlook completely. Check Task Manager for a leftover OUTLOOK.EXE process.

3

Open %localappdata%\Microsoft\Outlook in File Explorer and move the .ost file to another folder as a safety net.

4

Start Outlook. It builds a new OST and re-downloads the mailbox. On 38GB this can take a few hours on office bandwidth.

5

Once the new file is stable for a week, delete the old one you set aside.

You can also cap the rebuild size. File, Account Settings, double click the account, drag the Mail to keep offline slider down to 12 months. That alone usually cuts an OST like yours by more than half.

Moved the OST out and let it rebuild with the offline slider at 12 months. New file is 9GB and Outlook opens in seconds. Marking this solved.