Example mail and reply email:
Hi.
Herbert said that I should not paint again because I could hurt my wrists.
What do you think?
Alice
Hi Alice,
Herbert is on vacation and has no medical skills anyway.
I suggest to ignore him and to listen to your doctor instead.
Brad
Reply email with standard quoting:
On 22.08.2013 13:33, Alice wrote:
> Herbert said that I should not paint again because I could hurt my wrists.
Hi Alice,
Herbert is on vacation and has no medical skills anyway.
I suggest to ignore him and to listen to your doctor instead.
Brad
Note 1:
The quoted parts are prefixed with "> ", which is commonly recognized by standard email clients. An additional quotation level adds yet another "> " prefix.
Note 2:
Modern email clients properly quote the texts from previous replies automatically.
Note 3:
Common email clients typically use these prefixes to display the quoted parts in a different way to make it easier to tell them apart from the actual reply text. For example:

Note 4:
Any information not addressed by the reply, and any information that is not important for the reply has been removed. Optionally replace removed texts with {{[..]}} (to indicate that there was some more text previously).