Google App Engine allows for multiple version to be deployed concurrently, one of which is the default.
You can access non-default versions via HTTP (and apparently also via XMPP) using a special URL.
Is it possible to send email to a non-default version as well?
I believe it is not a supported feature since the Mail documentation about this aspect is missing and there's an open issue here.
Anyway, if the mails are handled like XMPP addresses
Each version of an app has its own set of XMPP addresses in the following format:
anything@version.latest.app-id.appspotchat.com
If App Engine receives a message for an address in this format, it is routed to the corresponding version of the app (if that version still exists).
you could try with:
foo@version.latest.app-id.appspotmail.com
EDIT: This post confirms that you CAN'T do it.