When cleanup and compacting still leave a huge file, splitting divides one oversized PST or OST into several smaller PST files. It sidesteps the size limit and lowers corruption risk. It does not delete mail, so the total volume stays the same.
When all the mail is needed and the file is 40 GB, split it. Split PST File reads a large .pst or .ost and writes several smaller PST files with Outlook closed. 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 and writes a CSV report of each part.
Splitting does not reduce the total amount of mail. It turns one file over the limit into several files below it, each faster to open and less likely to corrupt. The source file is read-only during the split. A PST viewer opens the parts without importing anything. A pst to mbox tool moves the same mailbox into the format Thunderbird reads natively.
How to reduce your Outlook mailbox without deleting emails?
You can shrink the working file without losing a message. Archive old mail to a separate file on another drive, move large attachments to folders, or split the PST into smaller parts. These options keep every message accessible.
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. Split the active file into smaller PST parts. For messages you want fully out of the file, you can save an Outlook email as PDF. If the years you want to move off the main drive still live in a Microsoft 365 mailbox, an Outlook email backup writes them into a separate PST.
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.
If Outlook will not open the file, open a PST file without Outlook to pull data out first. Delete or archive the largest items and compact. If the file misbehaves, run Scanpst.exe (the Inbox Repair Tool) before compacting or splitting it. On a legacy ANSI file near 2 GB, split it or migrate to Unicode. You can also move a PST file into a Microsoft 365 mailbox.
Pitfalls when reducing PST file size
Empty Deleted Items and Junk before compacting. Deletion alone does not shrink the file, and an OST does not behave like a PST. Allow enough time for a large compact, and move an archive to another drive when disk space is the goal.
✔️ Compacting before you empty Deleted Items.
Empty Deleted Items and Junk first, then compact. With those folders full, the file barely moves.
✔️ Expecting deletion alone to shrink the file.
Deleting mail leaves the .pst the same size on disk until you compact it. Microsoft's guidance is to empty Deleted Items and then run Compact Now.
✔️ 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. 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 will not produce the same shrink. Shrink the mailbox on the server.
✔️ Leaving the archive on the same disk.
A new .pst next to the original frees mailbox room but not disk space. Move the archive to a different drive.
✔️ Letting the file grow to its size limit.
On a PST over 50 GB thread, suggested fixes included repair, compact, or split. Act before the warning appears.
Split PST File
