There are some basic conventions to replying to a mailing list that make it easier for everyone to follow email threads.
1. Reply-to-all when posting to a message
Not everyone who has posted to the mailing list is subscribed to that mailing list, or they have the mailing list in a separate email. If they have posted a message from their email account then they will receive all followup messages and will be able to easily follow the email thread without searching the mailing list for followups. Most mailing lists filter repeat messages so you need not worry about them receiving two copies of the email.
2. Trim the quoted message
Trim the portion of a message you are replying to to just the part you are replying to instead of quoting everything the previous poster said. Often messages have lots of other information, code, etc that is no longer relevant to the discussion and end up making for huge messages that are difficult to sift through to find the relevant reply.
3. Do not top post
Top-posting is the practice of replying to an email and placing your text at the top of the reply, before all other text. We prefer to use bottom-posting because it is the easiest way to maintain a coherently flowing email conversation. The only time it is suitable to top-post is when introducing text such as "here is an interesting email discussion for you".
It is best to put your comments or answers directly below the relevant portion of text you are replying to. Eg:
I hate pie!
> We're well too. Please bring a pie
>> Hello. Great! How are you? We'll be here on the 4th. >> What do you want us to bring?
>>> Hi. How are you? When are you coming to our house? ---
>>> How are you?
>> Great! How are you?
> We're well too.
>>> When are you coming to our house?
>> We'll be here on the 4th. >> What do you want us to bring?
> Please bring a pie.
I hate pie! ---
Now imagine if there were multiple people and the discussion diverging into multiple directions and threads.
4. Be polite
Humans by nature don't realise how much they depend on seeing facial expressions, voice intonations and body language to determine the emotion associated with words. In the context of email it is very common to misinterpret people's emotions based on the text alone. English subtleties will often be misinterpreted even across English speaking nations, and for non-English speakers it becomes much harder. Without the author explicitly stating his emotions, assume neutrality and respond politely.
5. Obey the conventions
Common courtesy would mean you'd respect the mailing list conventions you are part of if you want to contribute.