PST file too large? Limits, symptoms and splitting

The size limits

The Unicode limits are defaults: an administrator can change them with the MaxLargeFileSize and WarnLargeFileSize registry values. Raising them does not make a huge file faster or safer.

Symptoms of a PST that is too big

• Outlook starts, searches and switches folders slowly, or freezes.
• Messages say the file has reached its maximum size, or new mail cannot be moved into it.
• The file becomes damaged more easily, and SCANPST takes hours or fails.

As a rule of thumb, archives become unpleasant well before the limit; keeping each PST to a few gigabytes keeps Outlook responsive.

Make it smaller: split it

Split PST cuts a large PST into smaller ones in your browser:

• by year or by month, using each message’s date,
• by top folder (Inbox, Sent Items, …),
• or by size only, filling parts up to a limit such as 2 GB or 10 GB; a size limit can be combined with the other choices.

Preview the parts for free first: every file, what it holds and about how big it will be. Each message goes into exactly one part, and every part is checked against the original before you save it. The original PST is not changed.

Other ways to shrink a PST

• Compact it in classic Outlook: File › Account Settings › Data Files, select the file, Settings › Compact Now. This reclaims space from deleted items but does not split anything.
• Remove duplicates: repeated imports and merges leave copies of the same mail. Remove duplicates counts them for free and writes a clean copy.
• Save attachments elsewhere and delete them from old messages, if you no longer need them inside Outlook.

Frequently asked

Can I split a PST without Outlook? Yes. Splitting reads and writes the PST directly; Outlook is not needed, and it works on a Mac or Linux too.

Will the folders be kept? Yes. Each part keeps the folder structure of the messages it holds.