Important: It is strongly recommended to have a mailbox size lower than 50 GB.

If a mailbox size is bigger that limit, Outlook and OWA can start working slowly or even stop responding at all.

In Outlook Backup service there is also a hard limit set for creating and restoring PST files - 100 GB.

By default Outlook application is able to open PST files that are not more than 50GB, however it is possible to increase this limit. Read Knowledge Base article How To Increase Size Limit Of Your .PST And .OST Files? for more information.

If you need to create a PST back up of mailbox larger than 100 GB, you will need to divide a PST creation by date and make at least 2 PST files, so they will be, for example, both about 50 GB.

To do this, navigate to Service > Outlook Backup and follow those steps:

1) In Outlook Backup click Create Backup.

2) Choose One-Time and uncheck Full backup button.

3) Set a Date range to be limited approximately to the middle of your mailbox usage time period.

4) Add necessary mailbox to backup.

5) Click Create Backup.

Create Backup

6) Once the backup is created, perform steps 1-5 again, but setting a Date range from the middle usage time period until the current date.

Create one more backup

Note: Outlook Backup will also fail to restore a PST larger than 100 GB, so you will need to make a restore with PSTs divided into smaller parts.