Yahoo Mail has a feature that would automatically quote the original message while replying or forwarding a message. This is useful when you want to highlight the original mail during replying or forwarding a message.
Login to your Yahoo Mail account and select Mail Options from the Options drop down.
In the Options screen, navigate to General options.
To automatically quote the original message, navigate to When replying & forwarding and tick the checkbox with label as Quote the text of the original message.
Message without quote.
Message after enabling Quote.
Depending upon on your needs you can turn on or turn off the include original message feature.