When cleanup and compacting still leave a huge file, splitting is the escalation path. It divides one oversized PST or OST into several smaller PST files, which sidesteps the size limit and lowers corruption risk. It does not delete any mail, so your total volume stays the same. You just hold it in manageable pieces.
Sometimes there is nothing left to delete. The mail is all needed, the file is 40 GB, and it is one folder shy of trouble. That is when you split. Split PST File reads a large .pst or .ost and writes several smaller PST files with Outlook closed. You choose the rule. By Size uses a threshold in MB or GB, set to 1 GB by default. By Date splits per year, month, or week. By Folder gives each top folder its own file, and By Sender groups mail by who sent it. It can exclude Deleted Items and Junk while it runs, and it writes a CSV report of each part.
Drag messages into a new .pst folder by folder. It is slow, easy to miss items, and it often leaves duplicates behind to sort out later.
Point it at the file, pick a rule, and it writes several smaller PST files while the original stays read-only.
Be honest about what splitting does and does not do. It does not reduce the total amount of mail you store. What it fixes is the single-file problem: one file over the limit becomes several files well under it, each faster to open and less likely to corrupt. The source file is read-only during the split, so the original is never touched. If you only need to read the parts afterward, a PST viewer opens them without importing anything.
How to reduce your Outlook mailbox without deleting emails?
You can shrink the working file without losing a single message. Archive old mail to a separate file on another drive, move large attachments out to folders, or split the PST into smaller parts. All three cut the size of the file Outlook loads every day while keeping every message accessible.
You can shrink the footprint without deleting a single message. If keeping everything matters, combine three moves. Archive old years into a separate .pst and store it off your main drive. Save bulky attachments to disk and remove them from the messages. And split the active file into smaller PST parts so no single file is enormous. None of these throw mail away. They just stop one giant file from dragging Outlook down every time it loads. For the few messages you want fully out of the file, you can save an Outlook email as PDF.
What to do when Outlook says the file reached its maximum size
If Outlook warns that the data file is full, you cannot move or send items until you make room. Delete large mail and compact, or run Scanpst.exe if the file is unstable, then split or migrate the data to a Unicode file. On an old ANSI file, splitting is usually the real fix.
Hitting the wall is a real crisis, not a warning you can ignore. Outlook's own message says the file has reached its maximum size and tells you to select items you no longer need and permanently delete them. If Outlook will not even open the file, you can open a PST file without Outlook to pull data out first. Then delete or archive the largest items and compact to buy room. If the file misbehaves, run Scanpst.exe (the Inbox Repair Tool) before anything else. A Microsoft moderator handling a mailbox stuck past its limit gave the same order. First check format compatibility, then repair the file before you compact or split it. On a legacy ANSI file near 2 GB, do not fight the limit. Split it or migrate to Unicode.
Pitfalls when reducing PST file size
Most failed attempts trace back to the same mistakes. People compact before emptying the trash. Others expect deletion alone to shrink the file, or treat an OST like a PST. Get the order right, set realistic time expectations, and the file behaves.
✔️ Compacting before you empty Deleted Items.
Run Compact Now with a full Deleted Items folder and the freed space is not actually free yet, so the file barely moves. Empty Deleted Items and Junk first, then compact. This is the number one "it did nothing" complaint.
✔️ Expecting deletion alone to shrink the file.
Deleting mail leaves the .pst the same size on disk until you compact it. Microsoft's own guidance is to empty the Deleted Items folder first and only then run Compact Now. Clearing mail and rewriting the file are two steps, not one.
✔️ Starting a huge compact during work hours.
On a Technibble forum thread, one tech reported a plain compact of a 2 GB file taking around six hours, and rebuilding a file of about 6 GB into a fresh PST brought it down to 2.5 GB. Run big compacts overnight and do not interrupt them.
✔️ Treating an OST like a PST.
Compacting an .ost the same way and expecting the same shrink leads to that 16.8 GB versus 6.4 GB gap. Shrink the mailbox on the server, not the cache.
✔️ Leaving the archive on the same disk.
Archiving to a new .pst that sits next to the original frees mailbox room but not disk space. Move the archive to a different drive if disk space is the goal.
✔️ Letting the file grow to its size limit.
Once a PST nears its cap, a quick compact no longer saves you. On a PST over 50 GB thread, the suggested fixes were repair, compact, or split, all slower than trimming earlier. Watch the size and act before the warning shows up.
Split PST File
