The manual seems pretty clear:
The REPLYTO keyword can be used when you want any replies to the e-mail to be delivered to someone else. The syntax is:
The address is any of the valid electronic mail address formats â€“ see the To Address above for address syntax information.
where one of the TO address forms is:
(address1 address2 â€¦) multiple addresses are enclosed with parentheses separated by at least one blank or a comma
Any reason YOU could not read the manual yourself?