Can I send a chat or conference invitation by mail when the user is offline?
Either your client must send the mail then or you write an Openfire plugin, which sends the mail.
But you need to connect your plugin to a Mail Server.
The easier solution is probably to just wait for the user to come online and then let him receive the invitation.
(Put an empty String in the body of the XMPP invitation message, there’s a bug in Openfire, that it doesn’t store empty-bodies messages).