I mirrored my Google Apps email account by setting up a rule to forward to my Microsoft-hosted Outlook.com account. It uses the same spam filtering system as Hotmail. A serious problem happened when perhaps 10 spam messages over a few days made it through gmails spam filter into my inbox, and on to my Outlook.com account.

Sadly when my gmail inbox started filling up with bounce messages from the MSN Postmaster, I realised that Outlook.com blocked my gmail account for spam abuse - SYSTEM WIDE. I was completely unable to send ANY mail to ANY Hotmail or similar Microsoft-hosted email service.

Dealing with the MSN Postmaster team was very frustrating. They told me to go to complain to gmail instead.

TL;DR: Don't forward all your email to Hotmail because their spam filter is so crazy, they'll eventually block your email address from mailing into Hotmail hosted servers again.