Just for the sake of completeness, an hourglass after the message means it is queued for sending, a single checkmark means it was accepted by your server, an exclamation mark means any of the recipients had produced an error and a double checkmark means no error was received and at least one of the clients got the message.
It would be great if avatars could be listed instead of a double checkmark similarly to how other messaging apps work.
The following issues are also loose related (for indexing purposes):