Yahoo (and others) use an algorithm (one they do not disclose) to determine what goes into the bulk mail folder. So, technically, there is nothing you can do to prevent this. You should create a Yahoo account and do some testing with different subject lines and body text. Don't put any $ or pricing information in the subject.
I had the same problem with my automatic mailer. In some cases you need to put in the FROM or TO mail field of the mail object, a valid e-mail address of the domain from which the e-mail is sent.
i.e. if your domain is Adomain.com then one of the two object fields(FROM in your case) must be amail@Adomain.com. I had this problem with Jmail..you can try it out
yes i had this problem on my intranet at work.
If i did not supply a valid email address it would get
sent to the junk mail folder.
I would assume other email providers have this same type
of filtering set up as well .