This is one of the things that drives me crazy. If you are going to be sending out emails do not use hotmail.com or yahoo.com or even your isp email address. Use your own domain email. If you want you can use a generic account in your own domain. When i submit to search engines i do not use my main account, instead I use email@example.com and that way any spam goes to that account.
This tip makes you much more professional in the eyes of your customer.