I'm not sure what you're trying to achieve here. The way I read your post, it's not a bug at all. You cannot send mails to XMPP-contacts (including google talk) only because XMPP-IDs contain the @ character.

my google talk contacts, automatically imported from google as soon as i set up my account, include a lot of jabber-IDs that are totally different from the person's mail address. sending a mail to these IDs just wouldn't work, so the N900 mail client must not offer them as a mail address.