Hey everyone!
I'm currently working on a tenant-to-tenant migration where we're merging mailbox content from one tenant to another. Some of these mailboxes in the new tenant are exceeding 50 GB, and while that fits within the 100 GB mailbox limit, I'm running into issues with the OST file due to using cached Exchange mode.
From what I've gathered, it seems like the OST file can't be increased beyond 50 GB based on the MaxLargeFileSize registry setting that limits it to that size. I've tried some registry tweaks myself, but I couldn't get it to go over the limit.
I know this isn't ideal, but it would really help in our current situation until we can archive the old emails properly. Has anyone had any luck extending their OST file past the 50 GB mark? Thanks for any insights!
5 Answers
I think I've been able to allow OSTs to grow above 50 GB using registry edits—there’s a setting for the size warning and another for the size limit itself. The key is the MaxLargeFileSize setting. But I’ve heard some say it might not work in versions past 2016, so it could depend on your setup!
If you want to increase the size limit of your OST file, you need to also make sure you have the right version of Exchange license—like Exchange Plan 2 (100GB). I’ve had success with that approach in the past!
I actually did manage to get an OST file to exceed 50 GB once for a client. But honestly, I wouldn’t recommend it. Outlook became unstable, treating the large OST as corrupted, which led to heavy scanning and rebuilding. It even ruined some SSD drives! Definitely proceed with caution if you go this route.
I researched this extensively and found that it’s virtually impossible to extend the OST file size beyond 50 GB. The best workaround I found was to reduce the sync duration for cached mode instead. Not the greatest but might be worth a shot!
Looks like there are some guides out there that claim to help with this. One that I saved ages ago was pretty useful: it talks about increasing OST file limits using the registry editor, so you may want to check it out!
Yeah, just keep in mind that doing this could lead to more profile corruption issues than normal. I'd weigh the pros and cons carefully.