It looks like if you have a Yahoo email account, they are greylisting their emails servers and causing a lot of mail to be refused.
Here's an article on greylisting:
http://en.wikipedia.org/wiki/Greylisting
and from the Yahoo Answer section:
http://ca.answers.yahoo.com/question/in ... 658AAAh0QY
http://answers.yahoo.com/question/index ... 547AAf54ah
My personal opinion is to get another email account such as the one from Google. I've been using it for several years and they offer generous disk space (almost up to 3 gigs) and good spam/virii filtering.
Google Mail is by invitation only. If you require one, private message me on this forum.