Importing a Large PST file Into Outlook Classic? Note What to Expect

If you are importing about 25GB of PST file into Outlook Classic, here’s some notes for you¹.

• Remember to run scanpst.exe over the exported PST several times before you import. It’s usual for scanpst to take several passes. Go until it finds no problems. This is a good practice to do especially on large PSTs.
• Also remember to increase the size of the PST via registry updates to make sure Outlook can handle PSTs of upto 100GB (50GB is the default), if you expect the final size will be bigger than 50GB. Of course, there are risks to using a PST greater than 50GB, be aware of them.
• On a 1Gbit line with SSD on the computer, it takes about 30 minutes to fully import the PST locally.
• It will then take anywhere from 4 to 16 hours for Outlook to push all the emails into the Cloud. During this time, there might be hours where your inbox in Outlook WILL NOT show the latest emails. It looks like when importing such a large PST (we had about 100k emails in it), the synchronization process is so heavy that even inbox doesn’t show the latest emails. However, the web version and phone app will show those emails. If in doubt, always look at message trace in Office 365 admin portal.
• If you have another machine with the same Outlook profile (say a user with two computers, after the machine into which the PST was imported fully syncs up with the cloud, downsyncing the new emails to the other machine is pretty fast. If they have good bandwidth, the downsync will happen within an hour or two.
• It’s always better to do large imports during the evenings because Office 365 might throttle the import during the day.

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