A reply simply confirms that your email address is active, and therefore more likely to be reused. Dealing with spammers is the fastest known method of being robbed. Spam offers usually sound too good to be true, for the simple reason that they are too good to be true. Many spammers use service providers in Eastern Europe, though the email may be falsified to appear to come through a reputable source. Once you pass your credit card details, your account will be drained very fast and efficiently, with little chance of recompense.
Many browsers automatically add the 'greater than' symbol to the left of each line of quoted text.
It distinguishes quotations from different senders, helping respondents to avoid errors in attribution. It is rarely necessary to quote a message in its entirety - just enough so that the recipient knows what you are referring to. In general, email is quite informal, but a business posting would avoid too many abbreviations and would exclude slang. Capitalization would follow written rules. If replying to business communication, be guided by their style.