If you’ve got a lot of email in Outlook on Windows, you’ll sooner or later come across a rather cryptically worded error message that doesn’t really clearly say what the problem is.
The error message is:
The Sent Items folder contains the maximum number of allowed items. Archive or move some items to another folder. Until space is available
The problem with the error message is that it’s got nothing to do with the Sent Items folder, and more that you’ve hit the storage limits of your PST file in Outlook on Windows. Practically, you probably don’t want them getting much bigger than this, but sometimes you need to.
When you get this error message, you have a couple of alternatives – you can either change the amount of cached email that Outlook keeps, or you can tell Outlook that the PST is allowed to grow bigger.
To increase the maximum size of the PST file, launch Regedit and go to:
Have a look in here and see if there’s a key called either MaxFileSize or maybe MaxLargeFileSize
If they don’t exist, create them (I’m not sure which one is needed, I will test this at some stage in the future), make it a DWORD and set the value to 102400 (in Decimal) which equals 100 GB. If you don’t want to double the size, you could always pick a value between 50 GB and 100 GB.
Quit and relaunch Outlook and the issue should be solved.