Outlook Oversized OST File Problem

 I have an Outlook 2010 user (whose mailbox size is over 50GB) on Exchange 2010. He uses Outlook 2010 in "cached mode".
 Today there was an error about OST size.  I checked the size of OST file and it was very close to 50GB.
 I understand that maximum size of OST or PST on Outlook 2010 is 50GB.
 For now I disabled "Cached mode" in Outlook 2010. But I prefer to run Outlook in Cache mode just in case Exchange Server is not available, at least the user would have access to all previous emails & folders.
 So what is the best solution here?
(1) Simply disable Cache mode or
(2) Change registry value "somehow" to allow OST file size bigger than 50GB if that is possible?

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AmitIT ArchitectCommented:
Yes, you can change the registry value on client side and increase the limit.
follow this

However, I recommend to decrease mailbox size.

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Alexei KuznetsovMicrosoft Outlook MVPCommented:
Although you can allow OST bigger than 50 GB, I'd recommend you to either disable Cached mode or archive old mailbox data reducing its size.
Lee W, MVPTechnology and Business Process AdvisorCommented:
You can increase the limits but advise the user Outlook may become slower.

AS per https://support.microsoft.com/en-us/kb/832925:
    Click Start, and then click Run.
    In the Open box, type regedit, and then click OK.
    In the left pane, expand My Computer, and then expand HKEY_CURRENT_USER.
    Expand Software, and then expand Policies.
    Expand Microsoft, and then expand Office.
    Expand 11.0 for Outlook 2003, 12.0 for Outlook 2007, or 14.0 for Outlook 2010, or 15.0 for Outlook 2013, and then expand Outlook.
    Click PST, and then right-click MaxFileSize in the right pane.
    Click Modify, and then type the value in the Value data box.
    Click OK.
    Right-click WarnFileSize, and repeat steps 8 through 9.
    Right-click MaxLargeFileSize, and repeat steps 8 through 9.
    Right-click WarnLargeFileSize, and repeat steps 8 through 9.

Note You may have to create the registry values if they do not exist. If the registry values do not exist, follow these steps to create them.
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sgleeAuthor Commented:
Thanks for the suggestions. The consensus seems to be against going over 50GB in OST file in Cached mode.
Has anyone maintained OST file >  50GB?
How slow they become?
Other than slower performance in Outlook, what else could go wrong or what else the user would notice?
AmitIT ArchitectCommented:
There is no issue in having OST of 100 GB also. However, it comes back to your machine Hard Disk speed. Normally, in client end machine you will have 5400 or 7200 RPM disk. If you have faster disk, you will get better performance. SSD much better. Remember, chances of OST or PST corruptions are high.

If you are using without cached mode, that will generate load on server and client end both. Reducing mailbox size or using archiving solution is best way to overcome this issue.
By changing registry value is the better approach  to increase ost file size limit. And if you ask about the performance of Outlook, of course it depends on machine to machine.
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