No, not possible, it is a fixed limit inside Messenger Plus!. Maybe one day Patchou will increase it a bit though.
As for a plugin, this would also be "a bit" impossible, because the plugin API isn't designed to respond to received text. There are ways around it though, it isn't completely impossible, but it requires a more advanced plugin then the usual normal plugins you'll see...
An example of how you can do this can be found here: "
Bot Developers Kit v1.00 beta 1 available!"
This plugin is kind of a "proxy" between MSN Messenger and Messenger Plus!/you, so it isn't a real standard plugin, although it gets loaded as a plugin...
Other possebilities are addons. In other words: not-Plus!-plugins, but MSN Messenger addons, which take over the sending and receiving just like Messenger Plus! does.
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