It's been discussed before and there is a Jira issue for it, but it's not solved and it's still a nuisance: the lack of a means to store incoming group notifications, offers, messages and the like. In other words, the SL viewer doesn't have an inbox!
Tonight I spent a few minutes online to get new clothing. After that I had to attend to family duties, so I left the laptop. I forgot, however, to logoff from SL. Which means that after 30 minutes SL logged me off automatically due to inactivity. No big deal, but there was one group notification with an attachment that arrived between me walking away from my laptop, and the client logging me off. When I returned to the laptop, I could only click 'ok' on this message, but I was unable to get a hold of the attachment it contained! So, without asking someone to (re)send the attachment to me it is now lost, unfortunately.
If you haven't voted for the Jira issue yet, please, consider doing so.
And, isn't the SL Viewer open source, these days? Anyone with coding skills care to take a stab at storing incoming messages in an inbox, or even in a folder in the inventory..?