If LiveAgent detects that the received email was sent by an automated script, or that it is in some way automatically generated email or a notification email, the rule actions Send mail or Send answer will not execute, but will add a message "Automatic reply to sender was not sent because the received email seems to be automatic notification" to the ticket instead.
This function has been implemented to prevent an uncontrolled loop of emails between automatic mailing systems: LiveAgent on one side and some other script on the other side.
Emails that will not be replied by LiveAgent rules include junk and bulk emails, and emails containing header auto-submitted: auto-generated.