here is how posterous works,
I make a A record of DNS of my domain (blog.mydomainname.com) to posterous IP address 188.8.131.52 , in the manage panel of posterous , i put my custom domain (blog.meetingly.com). Then i can reach my posterous blog http://mydomainname.posterous.com from http://blog.mydomainname.com url.
How posterous implements this? Is this a DNS level thing or web server level setting?
Thanks a lot!