OK, in that case the JOIN message you receive from others will look something like this:

:joininguser JOIN :#joinedchannelname

The exact format of joininguser varies depending on the server, but generally it will be a full nickname!ident@hostname.com. The colon (:) before the channel name is also optional. This is briefly described in the "Join message" section of RFC 1459 (section 4.2.1) however it's not very well explained.

As a note, you can use mIRC's debugging feature to see the exact raw messages it sends to and receives from the server. For example type /debug @debug and the @debug custom window will list all traffic.

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