Interestingly if I type back at someone with an older version of mIRC, like a v6+ they can see it fine but it doesn't seem to work backwards
This is probably due to Khaled's (almost) never ending attempts to maintain compatibility with older versions.
Of course, eventually, there will be versions that are just too old, but for the most part, if someone has a version that is that old then they are way overdue for an upgrade.