Server: CentOS7 with Dovecot 2.2.36 | Client: Windows 7 with Outlook 2010

After copying all mails from a POP3 account to an IMAP account within Outlook 2010, the .pst was around 65GB (had to edit the registry to overcome the default 50GB limit, increased to 80GB). The next morning, Outlook had frozen and it wouldn't re-open. Upon inspection of the pst size, it had increased to 80GB. I increased the pst max size again to 100GB. A few hours later, Outlook froze again and the pst size was now at the new limit. It should be noted that on the server, the Maildir folder is roughly 70GB.

Changing the settings in Outlook so that only headers are downloaded (File > Options > Advanced > Send and Receive... > All accounts > Edit > [highlight account] > Download headers only) seemed to have solved the problem. But, while not critical, it would be convenient to have a local copy of the mails as well.

My plan is to split the mail into a few imap accounts (user1_archive1@ and so on). I've been compacting (from Control Panel, assuming having Outlook closed speeds it up) for almost 3 days, on an off, and it has been reduced from 100GB to 80GB.

In case it affects anything, Dovecot has the Mailbox alias plugin activated to map the corresponding Outlook folders to Sent, Trash and Junk on the server.


  • Is there any way to speed up the compacting process?

  • Is there any way to make Outlook download the whole mails without the pst going haywire? Will splitting it into several smaller account help?

1 Answer 1


As far as I know, there is no option in Outlook to speed up the compacting process. If the PST file size is too large, Outlook will easily crash or freeze. Although we can increase the limit if file size, the recommended size would be around 10-15G.

Reference: You may experience application pauses if you have a large Outlook data file

We can split this PST file to several small PST files using the Export or Archive feature of Outlook.

How To Split A .Pst File In Outlook?

Oversized corrupted .PST Files

Archive items manually

  • I don't notice any slowing down inside Outlook, but yes I plan to split them into several imap accounts (after compacting ends... some day). But as I need to have all mails on the server I can't use the Archive option, which from I understand is only saved locally.
    – Bangaio
    Mar 4, 2019 at 23:48
  • Do you import this PST files to your IMAP account? You can first split this pst files into several small ones and then import them to your IMAP account one by one. IMAP account stores items in the OST files which can be synced with IMAP server. I'm guessing the syncing the large size of PST file cause Outlook freezing.
    – Perry
    Mar 6, 2019 at 2:56

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