Email reply to IMs bounce when offline

I sent an email reply to an IM I received, and got back a message:

This is the Postfix program at host data.agni.lindenlab.com.

I'm sorry to have to inform you that the message returned
below could not be delivered to one or more destinations.

For further assistance, please send mail to <postmaster>

If you do so, please include this problem report. You can
delete your own text from the message returned below.

The Postfix program

<mailtoim@data.agni.lindenlab.com>: unknown user: "mailtoim"

I realized this was probably because the person logged off before I could reply. I was able to send replies to someone who was still online just fine.

This bug should be fixed now. Glitch in the mail aliases table.

I'm curious, how long is it before the IM > Email session UUID expires?

Is it possible for two people who are both offline to message each other through through this system?

Yes. The IM -> email session expires after "a while" (not exactly sure, but I can look it up for you) if it's not used.

However, once established, two people who are both offline can communicate through the system; however, it's rather non-optimal (short messages only, and higher latency than just emailing directly).