Your PST file is full and Outlook has stopped sending and receiving. Here is the error explained, the registry fix that raises the limit, and the honest cure that stops it filling up again.
Every long-running Outlook profile ends up here one day. The PST data file quietly grows for two or three years, and then one morning Outlook refuses to send, receive or even delete mail because the file has hit its size limit. The video above fixes it with a one click registry script. This guide shows exactly what that script changes, how to do the same thing by hand, and - just as important - why raising the limit on its own is only half the job.
Not all PST files are equal. An old ANSI PST (Outlook 97-2002 format) has a hard ceiling of about 2 GB written into the format itself - no setting can raise it, and files that creep past it corrupt. A modern Unicode PST defaults to 20 GB in Outlook 2003 and 2007 and 50 GB in Outlook 2010 and every version after it, including Microsoft 365. Check your file type first: File - Account Settings - Data Files, select the file and open Settings. If it says Outlook Data File, it is Unicode. If it says Outlook 97-2002 Data File, you have the 2 GB emergency, and the separate guide below is the one you need.
The video's method is a small .reg script. You double-click it, Windows asks whether to add the information to the registry, you confirm, restart Outlook, and the limit is raised - no manual registry editing at all. Under the surface the script writes two DWORD values, MaxLargeFileSize and WarnLargeFileSize, into the PST key for your Office version. That is the entire fix: there is no patch, no add-in and nothing running in the background afterwards. Only ever run a registry script you trust - a .reg file can change any setting on the PC, so get it from the video's source, not from a random download site.
Prefer to see every change yourself? Press Windows + R, type regedit and go to HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST (use 15.0 for Outlook 2013, 14.0 for 2010). Create the two DWORD (32-bit) values if they are missing. Values are typed in megabytes: the 50 GB default is 51200, so 100 GB is 102400. Set WarnLargeFileSize to about 95 percent of the maximum - 97280 for a 100 GB limit - because Outlook starts warning and blocking at the warn value, not the maximum. Close the Registry Editor and fully restart Outlook. The step-by-step page below this guide covers the maths and the mistakes to avoid in detail.
Registry changes are reversible, but a sick PST is not a good patient. Before any fix, close Outlook completely - check in Task Manager that OUTLOOK.EXE is gone - and copy the PST file to another folder or drive. File location: File - Account Settings - Data Files - Open File Location. A clean file copy is the backup that every later step (compact, repair, archive) depends on if something goes wrong.
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. Do the cure while the bridge holds: archive mail older than a year into a separate archive PST per year, empty Deleted Items, then compact the main PST so the freed space actually leaves the file. Split this way, no single file ever needs a heroically raised limit again, Outlook stays fast, and a corruption one day costs you one year's archive - not your whole mailbox history.
The video runs the one click registry script live and shows the raised limit working in Outlook - watch it before you edit anything.
Watch Full VideoA Unicode PST defaults to 50 GB in Outlook 2010, 2013, 2016, 2019, 2021 and Microsoft 365, and 20 GB in Outlook 2003 and 2007. An old ANSI (Outlook 97-2002 format) PST has a hard limit of about 2 GB that no setting can raise.
Yes. The same MaxLargeFileSize and WarnLargeFileSize values govern both PST and OST files in classic Outlook, which is why the video's script is described as a PST and OST fix. An OST is only a cache of your server mailbox though, so a full OST usually means a very large mailbox, not a file you should archive by hand.
The setting itself is a documented Microsoft configuration, but bigger is not free: very large PST files open and search more slowly, take far longer to repair if they corrupt, and are harder to back up. Raise it enough to work comfortably - for most people 100 GB is plenty - and do the archive and compact cure instead of chasing ever bigger numbers.