Normally when I build sites I get access to the customers webhosting (which you need when making sites live) and just create a random email account using their own domain name just for the purpose of signing up to services like WordPress.com, Akismet, Email marketing services etc etc. I always then just forward emails from that email address to the customers other email address or one of my own emails...

For storing logins and api's I just save them in a .doc file along with any other logins and details I have of the clients.

This system has always worked for me.