I like to tell clients to keep the current and previous months' email in the Inbox and archive everything else.
When you archive, you effectively create a new data file, so Outlook doesn't have to strain against the weight of an oversize PST or OST file If you use Cached Exchange Mode in Outlook, you effectively take the data file from the server (PST) and cache it on your local machine (OST).
Microsoft also telling us that creating sub folders under inbox is a bad idea, because inbox folder will filled up quickly.
They suggest to make folder the same level as inbox and keep the inbox size under 4GB.
Typical user complain are about this sync problem are:– My email become slow When you look closely to the status bar in outlook you will see that outlook trying to sync folder at about 1-3.99GB.One of the issues is that even when you delete email from your Inbox, the size of the PST file may remain the same.If you're using Outlook 2010, you can go to Account Settings | Data Files and select the data file to be compacted.This can cause serious issues, especially when using PST files.Instead of just letting those Inbox folders grow to outrageous proportions, set up auto archiving so that your Inbox retains only a portion of those emails.