I've been hitting all around this for some time now but didnt know it was 20GB.
Microsoft Outlook versions earlier than Outlook 2003 have a 2 GB limit on the size of personal store (.PST) files. This is a common complaint by many companies who still use older versions of Microsoft Outlook and can't easily replace the use of .PST files for myriad reasons.
First, searching for information among several 2 GB .PST files can be challenging. The 2 GB .PST file limitation is also problematic, since many .PST files can show problems before they even reach the 2 GB limitation.
Unfortunately, upgrading to Outlook 2003 does not automatically make it possible to add more data to a pre-Outlook 2003 .PST file.
If you have an existing .PST file from an earlier version of Microsoft Outlook that is approaching or has already hit the 2 GB limit, you cannot "fix" it by simply installing Microsoft Outlook 2003, attaching the .PST file, and copying data into it.
Microsoft Outlook 2003 allows for .PST files of up to 20 GB, because the .PST file format it uses is different than earlier versions of Microsoft Outlook. For one, it allows native Unicode storage, whereas older versions of Microsoft Outlook do not.
If you've upgraded your email clients to Outlook 2003, the best thing to do is create an entirely new .PST file in Outlook 2003 using the new file format; then migrate the data from the old .PST into it.
Simply upgrading to Outlook 2003 will not solve the problem. You have to set aside the time and resources to actually migrate data from the old .PST format entirely.
Note that if the original file is larger than 2 GB or close to that size, and cannot be mounted, you may need to run Microsoft's Oversized PST Crop Tool to make the file readable again.
As a rule of thumb, I never let an "old-school" Microsoft Outlook .PST file grow much more than 1GB before migrating out some of the data. The new file-size limit presents little, if any, real-world storage issues.