I think we have found the email issue that some of you have experienced on EZDispatch. I found some helpful info on marketing land :
It’s happening, according to email expert John Levine, because Yahoo over the weekend implemented a change that basically tells all recipients, including Hotmail, Gmail, AOL, etc., to reject any mail originating from a Yahoo.com address if it fails certain tests. In this case, the test is that the sender email address domain must match the domain of the server actually sending the email — which isn’t necessarily the case if people use mailing lists or other software to send email for them, rather than using the Yahoo.com STMP servers themselves.
So, the problem applies not only to small businesses but to anyone using a Yahoo.com sending address to participate in a mailing list that uses other servers.