I had this problem of 530 SMTP auto error, (MS Outlook error message 0x800CCC78). The problem for me was that all the same settings worked at home where I have SBC Yahoo, but not at work. Turns out that ISPs are supposedly blocking port 25 for domains that ARE NOT theirs. In my case I was on a Sprint network, and they were blocking my sbcglobal.net domain. Presumably they do this for reasons of reducing SPAM... the ISP will only allow SMTP traffic for their domain users only, presumably as those are users they can easily cancel. This is a plausible explanation, seems to me,for why my Outlook works at home, but not on the road.

oh, I just proved the above, and found a workaround, at the same time. I can now access Yahoo smtp servers fine thru Sprintlink ISP (various googling says that Sprintlink doesn't block port 25, but that may be old info). The workaround: change the SMTP TCP port from 25 to 587. Found that tip here (http://help.yahoo.com/help/us/bizmail/pop/pop-27.html) and it works fine.