问题:

when to use http://schema.openid.net/contact/email and when to use http://axschema.org/contact/email in openid request.

回答1:

Unfortunately there is no standard schema for Attribute Exchange (AX). But the axschema.org seems to be the closest thing to it. To know which one to use, the best way I know of is to look for a "hint" about which schema(s) are supported in the OP's XRDS document. DotNetOpenAuth does this, for example, and has a good success rate. When the XRDS doesn't contain any of the AX schema URIs, you can send all three (yes, there are three) AX attribute URI formats to best ensure a useful response.

回答2:

Nowadays, it seems the preferred AX schema url is always schema.openid.net. The axschema.org domain has been down for almost a year (not that it's needed).

Refs: Chris Messina on OpenID list and StackOverflow: Google only responds to schema.openid.net.