Office 365 Outlook 2016 .OST 50GB Limit

Senior Member

I currently am in the process of building the Outlook Mailboxes on various computers, some of which stop at 9GB,14GB but mainly 49GB (49,000,000kb) on the OST file and say all folders are updated, I know why they stop at 50 due to the limit but i'm confused as to why the other profiles stop building.


From what I understand here are my options for the 50GB limit:


  • The default limit to the OST file is set at 50GB but can be changed in the registry to 90GB+ 
  • I can set the shared mailboxes not to download but will be slow to open them as it would be an online mailbox.
  • I can limit the mail to only come in 3 or 6 months opposed to before 12 months but results in emails from before that time not being stored.
  • I can split the very large group which seems to be causing the issues which will be a very long process and assign the specific group to different users.

Let me know if there are any alternatives. As I've been instructed by my senior that the first three options aren't acceptable and the bottom one will be a 2 week+ process when I have a whole office that needs computers now unfortunately. 


Thank a lot for reading and I appreciate any suggestions :)


4 Replies

The trouble you are having is that outlook and other modern mail apps use some type of encryption and compression.  There are limits to the size of the compression and encryption apps.  You are limited by math sadly.


Archive and Retention policies seem to be needed. 50GB ost needs are kind of ridiculous. The 9GB/14GB profiles are probably stopping at that because that is probably the size of their mailboxes. Not sure the use case here, but any Mailbox of that size and ost. file tied to that large of a mailbox is just asking for issues.

If you want to save your company data, then first you have to archive your .ost file. 

There are some ideas as:

1-> Compress your OST data to get compress around 10-20%.

2-> Use third party tool to split the file

3-> Save OST file to PST format

I suggest you to use third party tool is better solution to save your valuable company data.

Search tool from Google



Hi @bencole,


What license do you have in Office 365 ? If you have E3, E5, Exchange Online Plan 2 or others you can enable Online Archive and set the rule for archive to 5 years for example to older email's are available on Online Archive and if the user does not have 50Gb per 5 years you never have problem with your OST.


You can read more here