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How to Increase .ost mailbox file size in Outlook 2016 office365
How to Increase Outlook Mailbox Size to Get More Storage Space
Facing error “Your mailbox is over its size limit” or “You have exceeded the size limit on your mailbox”

By Default Microsoft sets size limit for the Outlook mailbox. When you reaches to that limit you may start receiving Warning or suggestion for the deletion of emails to download new emails.

The default limits are,

MS Outlook 2000 & earlier 2 GB
MS Outlook 2003, 2007 & 2010 20 GB
MS Outlook 2013 & 2016 50 GB size limitation

So, when you reaches the limit you can remove the old emails to make room for new or increase the mailbox limit. Yes! the mailbox limit can be increased.

Following are the steps to be followed,

* Close MS Outlook
* Goto Run
* Type regedit and press Enter
(Note: Any mistake in modifying the registry entries may cause serious problems. Be careful, Taking the backup of registry before modifying it would be a good idea)

* Expand the registry key to following,

For MS Outlook 2003
HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\PST

For MS Outlook 2007
HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\PST

For MS Outlook 2010
HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\PST

For MS Outlook 2013
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\PST

For MS Outlook 2016
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST

Here you need to create two new values

MaxLargeFileSize
WarnLargeFileSize

* Right click on the blank space
* Select New DWORD (32-bit) or QWORD (64-bit) (Depending on the bit version of your Outlook)

For Outlook 32-bit click on DWORD (32-bit)
For Outlook 64-bit click on QWORD (64-bit)

* Name it MaxLargeFileSize
* Now Right click again on the blank space
* Select New DWORD (32-bit) or QWORD (64-bit)
* Name it WarnLargeFileSize

* Double click MaxLargeFileSize
* Type Value data in MB (like for 100GB you need to type 102400)
* Select Decimal
* Click OK

* Now Double click WarnLargeFileSize
* Type Value data in MB
* Select Decimal
* Click OK

Close Registry Editor and Open MS Outlook