One of the biggest problems with our clients is to ensure that the legitimate emails they send out to their users/ customers do not end up in the SPAM folder. To ensure that as a website administrator you need to follow the bext practices guidelines provided by Yahoo and Google.
http://mail.google.com/support/bin/answer.py?hl=en&answer=81126
http://help.yahoo.com/l/us/yahoo/mail/postmaster/basics/basics-55.html;_ylt=Ahc6x60yB8n8JzvfVztu4QsIJHdG