If the local Outlook OST or PST file reaches 50GB, you will get the following message in Outlook:

The Outlook data file has reached the maximum size. To reduce the amount of data in this file, permanantly delete some items.

If you can't delete any items, you can add a value in the Registry to increase the maximum size. For example, to increase the maximum size for a PST or OST file to 100GB, add the following value:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST
MaxLargeFileSize (DWORD 32-bit) = 102400 (Decimal)

Restart Outlook and you now should not see the error message anymore.

Sidenote: Although the key in the Registry says "PST", it applies to OST and PST files.