difference between http://schema.openid.net/contact/email and http://axschema.org/contact/email

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

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.

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.