You ought to be able to send an IM to someone from your email even if you are not logged in by using the temporary address which SL gives when it sends IMs from SL to your email (looks like [email]kjshdg4mdsdj5jkkmlsf6lkdfdk7mlsdf8@im.agni.lindenlab.com[/email]). In other words, if you receive an IM in your email, it has an address which you can reply to which, if I understand correctly, is often valid for a few days.
My understanding is that you can use this method to send an IM to someone who is online as long as the message does not contain any unusual characters. I have actually seen it work.
But whenever I have tried it, the recipient gets a blank IM. They receive the IM online but none of the text appears, just an empty line.
I am wondering if this might be a Mac issue - something to do with the character set used in for example Firefox? I have not tried it with other browsers yet.