It's easy to use Posterous with a custom domain or subdomain.
But if my main website is on www.mydomain.com is it possible to have a Posterous blog on www.mydomain.com/blog, instead of blog.mydomain.com?
it's pretty easy actually. once you've logged into posterous, click on your site in the left hand panel && then click on settings on the right above post by email.
under settings there is a custom domain section. from there all you'll need to do is create a new cname record [ if you use cpanel this is pretty easy to do ].
here is posterous link as well :: http://help.posterous.com/how-do-i-use-a-custom-subdomain-with-posterou
hope that helps (:
To my knowledge you cannot set up a Posterous blog as a subdirectory. You can, however, set up a custom domain such as blog.example.com, then point www.example.com/blog to blog.example.com using htaccess, but depending on your situation that may not be ideal.
Here's an .htaccess file set up that will point a subdirectory to a subdomain: