I believe I have a problem with my web hosting company's mail server.
When I test my website from my development box (my PC) using my ISP's outgoing mail server I am able to successfully send an e-mail into a user's inbox in Yahoo. However, when I then migrate that code to my hosting company's server and use their e-mail server the e-mails go into Yahoo's spam folder. As far as I can tell both implementations are the same except they differ in use of the mail server.
The other differenec is that my hosting service requires that I use authenticated outgoing mail.
Both have the same e-mail sender so I don't think it's a DNS spam filter. Also my web site is brand new and hasn't had a chance to do bad things yet.
My hosting company assures me they are able to send e-mails from that server to use's Yahoo inbox. So I'm at a loss.
You need to read Yahoo's email FAQ as there are things they require in order to accept emails from a domain. From memory, they need the email server to announce itself in the headers and for the server to able to be resolved via a reverse lookup. Or something like that.