Looking at other posts (https://discussions.apple.com/thread/2664208?start=60&tstart=0) this is what I found to be a helpful answer for many:
This is by far the easiest and risk-free way to do this that I found. I've made a few switches in the past 12 months after changing from Windows, thinking Outlook would be the best option and now regretting it I wanted to change back. No need to buy add-ons or software, apply for gmail or anything else outside of your Mac.
Drag all your mail folders from Outlook onto your desktop and then you can bulk select them through the Import Mailbox option in the File menu in Mail.
Thanks JChp53!
JChp53 wrote:
There is a longish way to do this.
In Outlook, Select the folder you want to move (If there are folders within the folder, you have to select each individual subfolder, as the subfolders won't copy along with the main folder, only the messages in the folder will copy). Click and drag the selected folder (ex: inbox folder) to a new folder on your desktop. I couldn't figure out a way to select all the folders, so i dragged each folder and subfolder from Outlook into the folder on the desktop. Once you're done copying all the Outlook folders, open up Mac Mail. Go to File --> import mailboxes. Then select "Files in mbox format" and click continue. Then select desktop, and the folder you dragged all of your Outlook folders into. Hit "Choose." Mac Mail will import all of these folders, and when it's finished, will tell you that they are under the folder labeled "imports" in Mac Mail, and you just have to move all your subfolders back into their main folders.
Like I said, it takes a little while longer than I would like, but it worked for me and now I have all my old mail from Outlook in Mac Mail.
Hope that helps (and that I explained right)!