If they're set to receive IMs via email when offline, they'll see yourmessage outside of SL. If you never get a reply, they are eitherbanned, have quit SL or don't want to talk to you.
You still can answer an IM sent to you from inworld, if you receive it in your email. Just reply to it, and the person that sent it will receive an IM from your account....but only within the first 24 to 48 hours after the original IM was sent