I think that it couldn't be implemented in any way that would make sense for an irc client/servers to work. The load would be incredible and if you REALLY need to see offline messages, run a bot or use an IM program. Personally, I think trying to turn mIRC into an IM client goes against what mirc is, an IRC client. There's differences between an IM program and IRC client and I think trying to meld the two isn't a good idea. Ages ago I made a posting explaiing the differences between the two, but it was lost in the fire and I dont remember where I grabbed the info from, so I wont repost it
