When you reply to someone on a mailing list, your mail software will populate the "To:" address one of two ways: either to the person who wrote the post you're replying to, or to the entire list. This depends on how the list is set up.
There is a very simple way to determine if you're replying to a person or to the whole list (which might be bad if you're giving out your rates). Follow these three simple steps to avoid failing at mailing lists:
1) Read a message.
Chances are you know how to do this already. Great. Now you think to yourself, "Hmm, I'm a Drupal theme developer, I want to talk to this person." So you hit reply.
2) Check the "To:" field.
Before you even craft your obviously well thought out reply, LOOK AT THE "To:" FIELD. Does it say the name of the mailing list, or does it say the name of the person you're trying to reply to?
3) Adjust the To: field as necessary
That's all there is to it. Hope this helps.