Outlook Message Store Reached Maximum Size

Outlook may show “The message store has reached its maximum size” when its local Outlook data file (.ost or .pst) reaches the configured size limit. This is a local Outlook file limit; it does not increase the mailbox storage available on the mail server.

These steps apply to classic Outlook for Windows. They do not apply to new Outlook, Outlook for Mac, or Outlook on the web. Before editing the registry, try removing mail you no longer need and emptying Deleted Items, then restart Outlook. If the error continues, you can raise the local Unicode data file limit with Registry Editor.

Back up the registry

  1. Close Outlook.
  2. Press Windows + R, enter regedit, and press Enter.
  3. In Registry Editor, navigate to:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST

16.0 is used by Outlook 2016, 2019, 2021, and Microsoft 365 classic Outlook. 4. If the PST key exists, right-click it and choose Export. Save the backup file somewhere you can find it. If the key does not exist, create it in the next section. If an earlier part of the path is missing, create the missing keys in order: Office, 16.0, Outlook, then PST.

Increase the data file limit

  1. In Registry Editor, if needed, create these keys in order under HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook:
  2. Right-click Outlook, choose New → Key, and name it PST.
  3. Select the PST key. In the right pane, create or edit these DWORD (32-bit) Value entries. Choose Decimal when entering each value:
Value name Decimal value Purpose
MaxLargeFileSize 204800 Sets the maximum Unicode data file size to 200 GB
WarnLargeFileSize 194560 Sets the warning threshold to 190 GB
  1. Close Registry Editor and restart Outlook.

If Outlook is managed by your organization, its administrator may enforce the size limit through policy. Contact IT if you cannot change the values or they revert. Increasing the limit can reduce performance and does not add server mailbox storage; Microsoft recommends keeping the warning threshold at least 5% below the maximum.

If the error remains

  • Check whether the affected file is an .ost or .pst and confirm that the registry values are under the correct Outlook profile and version.
  • For an IMAP mailbox, confirm that messages are available in webmail before removing or recreating the local Outlook account. Recreating an .ost can require Outlook to download the mailbox again.
  • If the issue is actually that the mailbox is full on the server, ask your mail administrator about mailbox storage. Raising the local Outlook file limit will not resolve a server quota.

For Microsoft’s full explanation of the registry values and supported ranges, see Configure size limits for Outlook data files.