Half of all PST panic is aimed at the wrong file. An OST is a replaceable cache; a PST is the only copy of your mail. Knowing which one you are looking at changes every decision.
Outlook uses two data file types and they could not be more different in what they mean. A PST is a local archive - mail moved into it exists only in that file. An OST is an offline cache of a mailbox that lives on a server - delete it and Outlook rebuilds it by syncing again. Confusing the two leads to the two classic mistakes: archiving an OST by hand, and treating a PST as casually as a cache.
Exchange, Microsoft 365, Outlook.com and IMAP accounts keep the real mailbox on the server. The OST on your PC is a synced local mirror so you can work offline. That is why technicians delete a sick OST without blinking: close Outlook, delete the file, reopen, and it downloads fresh. Everything that matters - quota, retention, the real size problem - lives on the server side, and 'archiving' an OST achieves nothing at all.
POP accounts deliver into a PST, and every archive file you create is a PST. Mail moved there is removed from wherever it was and exists only in that file. Lose the file with no backup and the mail is gone - there is no server to re-sync from. This is why the size limit, the compact habit, the year-wise split and the backup discipline in this guide series all exist: they protect the file that has no twin.
The new Outlook for Windows is built around server accounts and handles PST files in a limited, staged way - support for opening and working with PST data has been arriving in phases, and it is not the full classic toolset. Practical guidance: keep doing PST surgery - raising limits, archiving, compacting, repairing - in classic Outlook, where the data file tools are complete and the registry values in the video apply. If you trial the new Outlook, leave your PSTs managed by classic until support fully lands.
The registry values MaxLargeFileSize and WarnLargeFileSize govern both file types in classic Outlook, which is exactly why the video calls its script a PST and OST fix. But the same number means different things: a PST at its limit is an archive that needs splitting; an OST at its limit usually means a huge server mailbox or an over-long sync window, and the durable answers are server-side cleanup or syncing fewer months - not a bigger cache. Raising the limit only moves the wall. An oversized PST opens slower, searches slower and corrupts more easily, and the repair takes longer the bigger the file gets. The registry change is a bridge that buys you time - the real cure is archiving old mail out, compacting, and keeping every PST at a sensible size.
The video's registry script raises the shared ceiling for both file types - watch it, then apply the right cure for the file you actually have.
Watch Full VideoWhile the account still connects, yes in effect: export the mailbox to a PST with File - Open and Export - Import/Export. An orphaned OST from a deleted account is a different, harder story - Outlook alone cannot open it, which is why exports should happen before accounts are closed.
It can be: OSTs hold shared folders, public folders,Teams and meeting data and deleted-but-synced items, and like PSTs they do not shrink automatically. If size becomes a problem, reducing the sync window or rebuilding the OST fresh is cleaner than fighting the file.
No. A full server mailbox is governed by its quota, and error 0x8004060C from the server will keep coming until mail is deleted server-side or the quota grows. The registry fix raises local file ceilings only.