The Apple Mail upgrade process in OS X 10.11 El Capitan has been problematic for many users. This screen shot shows my V2 folder compared to V3 folder.
Let’s look at what has changed and optons to fix a bad upgrade. This is an example of a V3 folder with a bad upgrade. If you have Time Machine or clone backup you are covered.
However, if you follow our next tip, you can put it to good use.
Items from an Exchange account are stored in the Outlook cache.
Cool Tip: There are several tools to check the space on your Mac’s hard drive.
In this other entry for example, we take a look at Daisy Disk.
To solve this problem, you can empty the cache in Outlook so that Outlook can download all the items from your Exchange account again.Then you might have noticed your hard drive’s free space rapidly decrease, mainly due to the relatively small size of the hard drives of these Macs.Now, if you take a closer look at your Mac’s hard drive, besides the usual suspects (like your music or photo libraries), you’ll notice that the Mail app can take a lot of space by the time you’ve been using your Mac for only a year.If you’re no longer using an app, you can delete these files to free up some space. In the Finder, press Command-Shift-G or click on the Go menu, selecting Go To Folder.In the resulting field, type or paste ~/Library/Caches/.If you cannot export from Mail in a prior version of OS X, you can use Emailchemy to convert these to mbox files that will import in the correct format.