You can reply to the sender of a message or you can reply to the sender and all recipients of the message.
Reply replies only to the sender of the original message. It does not include any other addresses that might be in the To: or Cc: fields. Clicking Reply opens a new mail compose page and populates the To: field with the name of the person who sent you the message.
Reply to All includes all recipients from the previous message. You can use this option if you want everyone who saw the previous message to also see your response.
Using Reply only preserves the body text. Attachments from the original message are not included in the reply.
Clicking Reply or Reply to All opens the Compose page, with the text from the old message preserved.
Depending on how you set your composing preferences, the text from previous messages may have a character showing the number of previous exchanges that have occurred since this text was written. For example if the reply is part of an ongoing conversation, and you have the option Include original message set and Use prefix checked, the text from each part of the exchange is shown with either a > or | prefix.
> Previous message
> > Message before previous message
> > > And so forth
Note: If the message is in a conversation, changing the Subject: line will cause your reply to appear in a new conversation.
With the message open, click Reply or click the arrow to select Reply to All. A new message window is displayed with the addresses in the header and the original message in the body.
Type your reply.
Note: If you add your signature automatically to your messages, your signature is automatically placed in the message according to the preference you enabled, either at the end of all messages in the compose window, or at the end of the message you compose above the quoted text.
If you want to change which part of the original message should be included in the reply, click Options on the compose toolbar. Your default option is marked but you can change it for this email only.
Click Send.