Forum Discussion
The default mailbox size for Exchange Online E3 and E5 users is increasing from 50GB to 100GB
I doubt Outlook's OST can handle a fully-synchronized 100 GB mailbox. You will have to reduce the amount of data you synchronize or connect online. Even the fastest SSD will have trouble with an OST larger than 20 GB...
Supposedly the OST and PST files can grow to be very large. By default, Outlook will only support 20 GB PST and OST files. However, if you modify the registry (we use Group Policy to do this, but you can manually do it as well), then you can increase the supported size. We have supported over 100GB PST files (OST files are the same file type).
Here are the registry keys to add (based on Office version), now have fun with that :)
HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\16.0\Outlook\PST
MaxLargeFileSize REG_DWORD Decimal Value: 102400 (100 GB)
WarnLargeFileSize REG_DWORD Decimal Value: 102400 (100 GB)
