Hi Subbarao,
The "Use Reply-To Address" refers to an option in options manager under the Email section in which you set a Reply To email address for all outgoing notifications. If in a mailbox, the Use Reply-To Address field is set to "Yes", then when maileater replies go out (if they are configured on that mailbox's rules), then it will use the Reply-To address from options manager, instead of the email address of the mailbox itself.
For example:
In options manager lets say you have the reply-to address set to "ServiceDeskReply@company.com"
In the maileater mailbox address is "maileater@company.com"
In the maileater mailbox you have the "Use Reply-To Address" set to Yes
If that mailbox has a rule which has a reply set up, such that maileater replies back to the sender to let them know it received the email and is or is-not creating a ticket, it will send that reply from the the Reply-To address that is set in options manager which is "ServiceDeskReply@company.com" rather than sending it from "maileater@company.com".
Hope this helps!
Thanks,
Jon I.