There is not enough free memory to run this program

Hi,
I am using Outlook 2013 and I often get this error when using it “There is not enough free memory to run this program. Quit one or more programs and try again.” And it affects both the Exchange and IMAP accounts with multiple accounts running.
Any ideas how this can be fixed?
Cannot expand the folder. There is not enough free memory to run this program. Quite one or more programs and then try again.
OK
This affects both the Exchange and IMAP account when you have multiple email accounts enabled.
