I do not wonder at all about that problem. The current label "Post" is indeed ambigous on what action it leads to.
Posting a new message is—right after reading others—one of the most important functions/features of an emailling group solution. I think it therefore should be more prominently presented.
I'd suggest to tweak the userinterface to make it more intuitionally/contextually understandable. See my attached mock-up.
I just would not name it "new message" for two reasons: Firstly, it again does not say what "new message" does: write, read, eat??? Not clear, not fast. Secondly, it already says "Message" above – doubling that would not be 'pretty' and also not helpful.
Apart from that question, and mainly for aesthetical reasons, I'd get rid of the forced First Letter Capitalization in the UI altogether – or at least in the groupnames. I think it looks outdated and does not add functionality, rather distracts attention.