[jeb]

Just setup htaccess on your original server to redirect to the new URLs. If you care so much about google juice, then you can spend a day doing this, instead of requiring posterous to solve the problem for you.

[fookyong]

how?

DNS will resolve to posterous' servers. It's out of your old server's control by that point.

Unless you're talking about different domains, e.g. moving from

mycoolblog.com self hosted -> myawesomeblog.com on posterous

in that case you could do what you said, but I don't think that's what the OP is talking about. the issue is when you use the same domain on both.