I use Gmail and Mail.app on a Mac too, and have been puzzled like you: Gmail orders the messages as "conversations" and when you send a message with Gmail, when it is sent back to you through the mailing list, then Gmail does not deliver again a copy to you… Indeed it assumes that you are already aware of it :-)

One solution (which I use) for being certain that your message has been received by the mailing list, is to configure your settings at 

so that you receive a confirmation message whenevr you send a message to the list.

Best regards: OK