I have a business email and a spam email hosted at Gmail. One was registered with the free Apps for Business before the offer ended, and the other is just part of a regular Google account. Somehow when I was configuring them to deliver to my Outlook account, the spam email account's messages started to be forwarded to the business account. I've Google'd the problem and most answers tell a user to go to "Accounts and Forwarding" and remove and links present there. This is not present on either account.
My only thoughts are that both of the accounts are linked on Google for easy switching (multiple login) and the business address is an IMAP account, while the spam is a POP. Interestingly, emails are initially present in the spam account before being forwarded to the business account.