From Second Life Wiki
When a Resident is offline, up to 25 messages will be stored and delivered at the next login. This limit includes inventory offers, group notices, group invitation and IMs. All messages above this limit will be capped and not delivered.
However, there is an IM to Email feature, which sends offline messages directly to the mailbox connected to your account. To activate it, go to Edit > Preferences (or press- ), go to the Communication tab and check the Send IM to Email checkbox. Press Apply and close the window with OK.
When an offline message is received via Email, this message can also be answered via Email again. These mails will be converted back to IM and sent to the right Resident. Note that the Email to IM feature expires 5 days after the receival of the IM. Messages sent afterwards won't be received anymore. Also note that the length of properly delivered Email replies is limited to 1023 bytes/single-byte characters. Messages exceeding this limited will be truncated to fit the limit.
Note that, due to, replies to offline emails currently can't contain special unicode characters.
Also note that offline IMs are only retained for 31 days and discarded afterwards. For example, if someone sends you an Instant Message and the next time you login is 33 days later, you won't receive it.
A conference is a private conversation with two or more Residents, who can communicate gridwide. It is similar to group chat, but more dynamic, since everyone on your friends list can be invited, as well as everyone where you got a calling card from.
To start a conference by
- selecting several Residents in your friends list ( +LEFT CLICK) and press "IM/Call" button.
- dragging a Resident's calling card into an IM window.
Once conference is started you can invite new Residents by dragging their calling card in the conference window.