[nielssp]

I'm pretty sure that's how most, if not all, instant messaging services worked 20 years ago... Was a feature, not a bug. The whole idea of sending an instant message. If you wanted to send a non-instant message, you'd send an email instead.

[machomaster]

That' not true. Icq and other messengers (don't remember the names anymore) had async messaging functionality and that was expected.

[grishka]

ICQ worked more like SMS. You could send a message to someone who's offline. It would get stored on the server and delivered when they come online.