When you choose to reply to message, a Message composer will open, with predefined fields:

• To: field will contain message sender’s address
• Subject will be added from original message, prefixed by “Re:” (or localized form for device active language)
• Original message will be added at bottom as quoted text

Reply all option appears if there are multiple recipients in the message. This is similar as replying to sender, however other recipients will be put into “To:” or “Cc:” fields in the message composer.